Permit me to digress. I realize that this thread has tipped heavily in the direction of grams-shaving, but I'll bore everyone that bothers to read this post a bit more with a curmudgeonly discussion of lateral wheel stiffness.
On a tandem, more lateral stiffness is better, and I'm not sure that you can have too much. The side to side forces operating on tandems are much more than a single bike, and it can be felt. You might interpret a laterally soft rear wheel as something else- a whippy frame, a soft rear tire, or whatever. If you value crisp, precise handling and response, stiffer is better. It's no mistake that sprinters prefer the laterally stiffest wheels that they can find.
A couple of scholarly works that touch on lateral stiffness can be found at:
http://www.duke.edu/~hpgavin/papers/...heel-Paper.pdf
http://www.sheldonbrown.com/rinard/wheel/grignon.htm
Gavin's paper is a forest of complex math (which I don't pretend to comprehend), but the interesting thing for us curmudgeons are the two graphs that relate lateral wheel stiffness to spoke diameter and spoking pattern. In all cases, the 14G spoke provides the most laterally stiff wheel. Interestingly, a 2X pattern gives the most torsional compliance in the plane of the wheel (that's the flex you get under braking or acceleration), and some folks feel like that is a good thing- it gives a more comfortable ride. I'll stick to a 3X pattern for my tandem wheels myself.
If you're searching out bling, definitely go for the exotic wheels with only a few spokes. 14G spokes do not have that degree of coolth. If you're looking for performance and sharp handling, stay with conventional wheels built with 14G spokes (I've veered over into personal opinion here....). Given the large forces operating on tandems, especially at the rear wheel, it's smart to prefer strength and reliability over light weight. Lose the weight somewhere else.
I've actually done the work in the real world, having built and tested wheels done both ways, and the difference is anything but subtle. On the other hand, I've been told by Jobst Brandt (among more than a few others) that I'm wrong. So there you have it.
Rick