Physics engines are slow and Tremulous already is pretty CPU-intensive. Not a good idea before a good rewrite of the renderer so it actually uses your videocard instead of your CPU.
The renderer uses the CPU and not the GPU?! I never knew that before.

Also, physics engines aren't really slow. (I have an AMD Athalon 2x 3800+ though, so I can't really judge their speed on average computers.

)
Furthermore, I'm sure the devs would be happy to rewrite the entire game into a new engine if we bought the engine and paid the devs to rewrite Trem. Otherwise we will just have to look for Q3 based games that have improvements and submit patches.
Edit: I just noticed the poll had a mistake. NS does
not use the Source engine. It uses the GldSrc engine. The Source engine is far superior to the GldSrc engine.