Not being a mountain biker I never thought about through axles before reading this thread. I have done a little reading about them and they seem to be stronger and stiffer but with some possible weight penalty. Seems like a marginal design gain that requires a large new investment.
Through axles appeal to me but I am not a big weight weenie and it seems like a good idea to use the best technology to keep the wheels on the bike. It seems like tandems should move toward though axles but my investment in wheels and tandem frames will keep me using my adequate steel skewers for some time to come.
Now if I wanted the lightest tandem I would use Ti skewers and a cable shifting double chain ring.