I read that there was a risk that T-Mobile would crack down. Apparently in the TOS they said something like use should be limited to the T-Mobile network (and such you shouldn't use it on wifi or whatever, but I checked and it worked fine on the computer).
Either way, as long as you understand that they miiiiight disable your access. At which point you paid $5-$10 for a month (or less) of MLB tv