Stop and think for two seconds. What happens if you increase enemy AI? Do you think they'll magically be able to live longer? How. They tend to die within a few scant seconds of being in your crosshairs. How does making them smarter really change that? Making them smarter may make them able to get out of your crosshairs better but they have such little life it doesn't make too much of a difference. So long as you can aim exactly where you want grunts will die quick deaths. The only real way to stop this, and therefore showcase AI, is to make weapons weaker (which no one likes), make aim less accurate (which people really hate) or to make enemies take much more abuse (which isn't very entertaining, either.)
Again, go ahead and mod UT3 for any or all of these variables and see if the "challenge" makes the game more entertaining or not.