John Carmack promised that Quake II will use MMX, but this wasn't done. That's why software rendering in Quake II is so boring, it doesn't even support colored lighting. In the other hand Unreal did use MMX, which allowed it to work in 32-bit color mode with colored lighting and a bunch of other effects.
IME Q2 also ran much slower on comparable hardware despite looking worse. The first rig I played them on was an unaccelerated P200MX with a paltry 32 megs of RAM and Q2 struggled at 320x240 while Unreal was slightly choppy but acceptable at 400x300.
Good lord, 400x300.
Mh, QuakeII without hardware rendering was nearly unplayable :-))