You just hit in on the head. Over in the tandem forums there are plenty of people spouting off on their preferences for tandems and posting to the effect of trying to validate the steel tandem they bought. Listen, except for very few teams most teams will never EVER race a tandem. Most of us will never do something like RAAM, or Paris-Brest-Paris. Most of us won't live like Dr. Breedlove:
https://ultracycling.com/sections/awards/hof/breedlove/

So does performance on a tandem really even matter? I think it matters more so than on any other bike. I agree with Santana that your tandem should ALWAYS be your best bike. Riding a tandem is goofy fun, but there are so many things on a tandem that are a complete compromise. Bike fit is magnified. It is so difficult to weight and unweight, or to stand on a a tandem because the stoker and captain are connected by the timing chain. You can't simply just stand or unweight by stopping pedaling whenever you want. You have to communicate and coordinate starting pedaling and stopping pedaling (unless you're on a DaVinci with independent pedaling) Stokers hate it when captains just start/stop without warning.

The reality is that at least half the cyclists that try tandems end up hating it. I love riding a tandem. My ex didn't enjoy it. She wasn't a cyclist and it wasn't her thing. She enjoyed mountain biking but never our tandem. My wife doesn't like the tandems. We have a "friend" tandem (a cheap Santana Arriva steel tandem) and offer to let all our neighbors and friends use it and borrow it. No one EVERY has used the bike. Not once. Our new neighbors said they might try it. The thing is it is very difficult for most stokers to give up control. While the average person has no problem riding in a car as a passenger the average person can't actually get emotionally comfortable riding as a passenger on a tandem. Its an interesting phenomenon. For many women that love riding bikes the freedom, independence and felling of flying of riding the bike is paramount and central to their cycling experience. Add to that dysfunction in relationships and what can happen when teams get tired, hot, and sweaty and many people don't have good tandem experiences.

However, if you actively get involved in the tandem community you will find that nearly everyone is absolutely willing to swap stokers and allow your stoker to see how they like riding on other bikes (carbon, aluminum, steel) and different makes. You'll get great feedback. A lot of stokers will share that they just aren't comfortable on road bars with "dummy hoods"). In reality you can use something like a Nitto Albatross bar for the stoker to get them more upright and still get a pretty significant aero advantage even on a stretched out bike like a Cannondale giving the stoker a long comfortable stoker compartment. Remember, the stoker is ALWAYS drafting on you, perfectly anyway. They don't need to be in a aero tuck.

The reality is that for Captains buying a tandem usually has to do with their budget, the components, and their image of themselves as a "serious cyclist." For most teams the perspective of the stoker has everything to do with comfort, they have a different tolerance for suffering with a given cadence to push through a climb, or to get in miles. The stoker wants to be comfortable and enjoy the ride, not typically push through for time. Captains and stokers seem to not do a very good job communicating about the bikes THEY buy together, which is interesting. Its always fun when a stoker gets off a nicer bike and tells her captain "I want one of those." Not everyone has the same budget.

Having the privilege to ride on a custom carbon Calfee or a magnesium Paketa is going to give a very very plush ride (nothing absorbs road vibration like magnesium, and Paketas and Calfees are both properly stiff and stiffness is critical to a tandems performance) but also reveal to someone who has only ridding on a steel tandem (cheapo or Santana or whatever) a huge performance/comfort gap. Stokers learn from each other the value of having a suspension seat post. Santana uses Tamer posts, which I once had with a titanium post, but now we have a Thudbuster ST which has less "stiction" in my mind (after talking with the designer). The guys at Tamer are great too, I've talked to them as well. Stokers love to commiserate about how terrible their captains are about announcing bumps or railroad tracks, but I think very few people who are captain's ever get the chance to be a stoker or vice versa. I think being a stoker is hard because you aren't in control, its a complete trust exercise. I think being a captain is very very difficult. Riding a tandem is exhausting emotionally and physically because of the concentration level is so much higher. You can't just stop mindlessly like on a single. If you don't support the bike with a stoker clipped in, the bike will fall over with her on it. You can't just simply misshift or "power shift" through your drivetrain to the same degree. On a tandem there can be up to twice as much force on the chain on the rear cogs, and shifting under load can be difficult. Teams have to learn to spin their cadence a bit faster to "unweight" the chain on the cog when slogging through a climb. Climbing on a tandem just plain and simple sucks. Many teams get very good at it. Very few teams learn to stand and hammer, as captain's tend to swing the bike around when standing without realizing it (think a grand tour sprinter). Coordinating climbing is tough.

However, I will say that I've met very very few teams that first have the means to afford carbon, titanium, magnesium, etc. yet still prefer steel tandems. I've met many teams for which the performance of their tandem has nothing to do with their tandem experience. However, as the weight of the team goes up, or the frame size increases, the appropriateness of steel as a frame material goes out the window in my opinion.

Anyone stoker will be able to swap with another stoker with nearly minimal adjustments to the bikes during a tandem rally. Stokers can ALWAYS ride lots of tandems. For Captain's its much harder. Captain's will trust their stoker to another captain before they'll trust another captain with their bike. Its funny that way. I've found the best place to try different tandems is via Craiglslist or through bike shops that are tandem dealers. Unlike singles, bike shops that sell tandems understand that tandem teams need long rides to sort through their opinions of the bike or build.

My advice ride as many tandems as your stoker can manage, and as well as a team. Ride steel, aluminum and carbon if you can manage it. Try a Paketa, at minimum just your stoker. There is a whole world of performance tandemming out there beyond steel. I'll leave my opinions about steel out of this, but steel tandems just make no sense to me. The stoker compartment is so absurdly cramped compared to a Cannondale. The frame is so much less efficient and climbing, which on a tandem is already a struggle, is exacerbated by wattage robbing frame flex. Steel tandems actually track less predictably and handle more "vaguely" which gets interesting on descents.

Considering that a Cannondale tandem can usually be had for $1000-1500 in most markets, I don't understand riding a steel tandem. Then again I've seen nicely equipped steel tandems with sell for around $400. Sometimes a bargain is hard to ignore, and a TON of fun can be had on a steel tandem, if the couple even takes to tandems.