I think the bottom line is that the low voltage T version CPUs are really only useful in applications where you must limit the TDP to a certain point.
I still haven't met a 1155 BIOS which can't underclock. That'll limit your TDP just fine (except perhaps at boot).
But if you were going to underclock to limit TDP, you'd be better off buying a Celeron or something in most cases. That'd limit your draw at boot too. So even if you wanted a T, you probably wouldn't want the 2100T.
I guess once you reach the lowest clock the T versions might have a slight TDP advantage, especially if you can't undervolt. And they're likely to give you slightly more peformance for the same power usage if you don't underclock all the way.
But by all accounts the difference is very slight.