I know there was a heavily mixed opinion on Shran joining the cast in Season 5, but I agree Red Ranger, I would have loved to have seen Jeff Combs as a regular.
I always particularly enjoyed those episodes he was a part of. I felt they really paid special attention to how they visually shot the character and Jeff always received the benefit of some of the best (not-so) subtle EFX work with the antennae in ENT. The "swirling" of his antennae when he took a swig of Romulan ale sitting at the table with Archer in "Proving Ground" was priceless. :-)