I joke to my wife about how much $ was spent to keep her bottom end comfy.
Both of our off road tandems have Thudbuster long travel posts. These work pretty well in the dirt for most bumps, but I still call out many things we are riding over.
The Thudbuster LT is nice also on account of being able to dial in spring rate and preload of the elastomers.
Shifting over to our road tandem, our first road tandem was a Co-Mo Speedster, it has a steel frame. On that bike we ran an aluminum seatpost and let the frame absorb road smootness, and called out the other stuff. That was a very smooth bike. Very nice.
As nice as the Seedster was, my wife never enjoyed our heft flexing the frame vertically at speed on flat ground. We then went with an aluminum framed Co-Mo Roadster. This bike had aluminum seatposts. We tried that and it wa ok but a little buzzy letting her feel the rolling road. To dampen this one of my mtb sacrificed a Dean Titanium post. This has proven to work well for us.
I guess the moral of this story is as others said, rider comfort. Because we had them in other bikes, our road tandem could be fitted and tested with a variety of different seatpost setups. The stoker made her own choice for each bike.
If you do go with a Thudbuster, be aware that with a rack or bags, the seat will move af and down to funtction. Also, unless it is plug and play for seattube diameter, buy the Thudbuster seatpost shims. They are and work best and are not cost prohibitive.
A lot of this decision will be based on what your frame is made from, how "textured" the roads are, and how sensitive your stoker is, plus how well you manage callouts. I would say Merlin hit it on the mark.
PK