I think it has proven out pretty well in practice that probably want both in the surface language. I know minimalists on the TF side of the debate have tried to make the case that you don't even need FDs in the surface syntax, but there are lots of places where it having a class with multiple directional constraints makes the code much, much more sane.
uestion yet? I thought MPTC was not considered usable without one of those, and neither is yet considered standard (with some good reason in the case of FDs).