Those other engines have shown us something impressive already. We have been able to play them. To see them run on our own rigs.
So far, compared to other games out there, the Doom 3 engine is nothing special at all. I suppose, as you say, it is capable of more....but we don't really know how much more yet. So far, it's just a next-gen engine that is, at best, a under achiever and at worst a sub-par engine.