It is true that what may appear to be an AI improvising cover is really part of a "canned" script by the level designer. From the participant's perspective, at least initially, they may have the same effect. But the more sophisticated participant will be able to figure out where the scenario is scripted and even use this against the 'bot opponents in the scenario.
In the case of F.E.A.R (my current, personal, favorite in this area), I can't see "under the hood" enough to know if there is a real AI there or just really, really, good scripts.
It is also true that in Halo, Half-Life 2, and many other games also have some interesting capabilities in this area. Some of the other fire-team based engines attempt to give squad-mates more realistic behaviors. Full Spectrum Warrior and First to Fight both tried this - only the former with enough to ensure commercial success.
I kind of don't care if there is a real AI there (just about the outcome - not the implementation) and, because I'm not really smart enough to solve true AI issues, kind of hope there isn't a real AI there.
The functionality is fascinating and it isn't to hard to discern the good implementations from the bad ones.
In some lesser implementation of this feature, the script or "AI" says the equivalent of "move to this point, duck behind this thing, and shoot over there". In a level that is very linear, those shots still (mostly) are coming at you and it is hard to tell the difference. When this technique is implemented on an open level, it is easy to defeat - just move out of the line of fire and your (dumb) opponent will keep shooting in the same direction while you flank him.
It a better implementation (F.E.A.R. appears to do this - not enough play hours to confirm though), the bullets reliably still come in my direction (darn it!) as I try to flank my opponent. The duck and cover maneuver may still be scripted but at least the opponent retains some situational awareness and response.
Lesser implementations also have only a few places on the level where this maneuver can be performed - the rest of the level looks "real" but isn't interactive or destructible and only a small-number of tables, barrels, etc. have been pre-defined to support this. Once you run thru the level/scenario a couple of times these are easy to identify and defeat - just maneuver about the level so that your opponent stays away from the objects they can use for cover.
In a better implementation (again F.E.A.R, COD2, and others) much more of the environment is interactive - every table can be flipped over, ever barrel can be moved, etc. There have been some interesting articles recently on the growing CPU demands of games and not just for the graphics (largely off-loaded to the GPU now) but for the "AI". I admit though that "AI" still deserves the quote-marks. It may just be a level with lots of loose objects and lots of scripts attached to those objects - there isn't artificial intelligence as the professional cognitive scientist would define it.
Still, whether the implementation is robust scripts or a genuine AI engine, if done well effect is still an admirable level of realism - I just wish I could get more time and access to see how it is done in the higher quality projects.