It's been suggested before, and I believe Timbo just doesn't want to put all that work into it. Another programmer is certainly welcome, however there are quite a few challenges involved with tremulous:
(1) Building bases. Do bots just build and rebuild in the same spot over and over? Are they supposed to have some 'intelligence' about building locations?
(2) Aliens. I imagine they'd need a very large re-vamp for all the abilities of the aliens to be implemented..
(3) Strategy. One of the best things about Tremulous I think, is all the different strategies, and new strategies not thought of yet. How can you expect bots to implement any of these?
Surely some stupid bots could be worked out... but for training, testing, or even coop, I don't think they could be made without a huge commitment of a programmer.
Also, I know absolutly nothing about how bots are made and implemented, which means I'm just kinda making up stuff here, but I'm guessing its fairly close to right. >.>