The bicycle industry has been fiddling around with bottom brackets as long as there have been bottom brackets, e.g., cotter pins, one-piece, square tapers (ISO & JIS), Octalink V1, ISIS, Octalink VII, ISIS II, Mega Exo, Gigapipe, etc... What a mess.
Personally, and like you, all 10 bikes in our stable -- to include all three tandems with their daVinci cranks -- have either a JIS or ISO square taper bottom bracket: simple, durable, and at one time easy to find and inexpensive even for the better quality brands/models. Well, if there's one thing that the bicycle industry seems to have a disdain for it's the status quo. So, all that said, to your questions:
1. What are the disadvantages of (square taper)?
It requires a certain amount of knowledge to properly spec. one for a set of cranks (e.g., to JIS for most these days & ISO for Campy and some other cranks) and to service, to include understanding torque specifications and the like. In practice, square tapers are often times over-torqued by shade tree bike mechanics and many so-called "bike techs" who actually got paid to ruin cranks. Removing cranks with square tapers is also difficult because it requires you to either: a) own and know how to use a crank puller, or b) be resourceful enough to discover those wonderful little things called self-extracting crank bolts. Finally, there were way too many affordable, long-lasting square taper bottom brackets on the market in a wide variety of sizes which really put a cramp on bottom bracket product line margins for those making them as well as crank manufacturers. Remember, the bicycle industry does not like the status quo: it's bad for business.
2. Are (Octalink, ISIS, etc) stiffer?
Actually, I'm told that they are and I've seen data that suggests this is true. Of course, I'm not sure just how big a deal this is for the average cyclist or tandem team vs. Magnus Backstedt or Alessandro Pettachi. Of course, stiffer is relative in that the added stiffness traded off durability with the first generations of Octalink / ISIS (and even the second generation compared to the better square tapers), and they're still getting the kinks out of some of the outboard bearing designs.
3. Are they lighter?
Actually, no... They all appear to be in the same ball park when you have similar materials and price points. There are high-end and expensive BBs of all types that are svelte and low-end models that are porcine. The latest and most highly integrated models are hard to compare since the manufacturer have succeeded in making you buy their proprietary bottom brackets to go along with their proprietary cranks because the spindle is no longer part of the bottom bracket (hence, the weight of a Mega Exo BB is clearly lower than square taper or splined which do include the spindle) and is, instead, part of the crank. Anyway, if you're bored you can find a pretty comprehensive list of the various pre-MegaExo type bottom brackets and their weights here at the
Weight Weenies Website.
4. Were any compromises made to move the bearings outboard of the bottom bracket?
IMHO, yes... The outboard bearing designs are perhaps the best example of how the bicycle industry's dislike for the status quo can sometimes get out of hand when an agreed to set of standards isn't put in place before companies go off and start spending big $$ on R&D and new product development. The very need for the outboard bearing is, in and of itself, a bit suspicious. At least one person who watches the industry even more closely than I do has suggested that the outboard bottom bracket designs are more about marketing and building moats around proprietary product features than they were about truly fixing the mess that was created during the Octalink & ISIS wars. The biggest problem -- noted by my friend -- is that these new Octalink and ISIS bottom brackets didn't look any different to the average consumer who never pulled apart their crankset where they could see and appreciate the changes.
So, from a marketing standpoint, it's kind hard to grab consumer interest in your product's unique and distinguishing features -- never mind charging a premium -- when they are essentially invisible and indistinguishable from one another. The outboard bottom bracket designs changed all of that because they were radical-new designs that broke apart the status quo of the bottom bracket design. Of course, we now have four competing proprietary outboard bearing designs: like life wasn't confusing enough when you first discovered that English & Italian bottom brackets were threaded differently.
5. Is this for real, or is it just a fashion fad?
Octalink is for real. ISIS is on the verge of extinction, and there's no way that the overall bicycle industry can support having more than one outboard bearing design: three of the major designs aren't going to make it... who will be left holding. Time will tell.
Bottom Line: With few exceptions, square tapers are more than adequate for the vast majority of cyclists. They have few REAL drawbacks for all but the strongest cyclists who have a compelling reason to eliminate all forms of drivetrain deflection, e.g., professional racers. Splined interfaces are a bit more idiot proof and help to preclude so-called "bike shop techs" or do-it-yourselfers from deforming square taper cranks by over-torquing during installation which may or may not be a good thing, i.e., it allows their lack of skill to go unnoticed a little bit longer similar to how threadless headsets no longer require as much "touch" to properly adjust compared to the older quill models. Good and skilled mechanics are a dying breed
Long-live the square taper. Hey, I own a torque wrench and I'm not afraid to use it.