At D3 I used the zone-press combo...made the NT five straight seasons, never went further than the second round (easily could have made the Sweet 16 a couple of times but lost some late-game coin flips in some tossup contests). The power of the combo has been softened with the new engine...the steals/turnovers generated were noticeably scaled back. I still think it performs at a level above just running the straight zone for generating turnovers, but the distinction isn't nearly as clear as it once was IMO.
For me, the main attraction/benefit of running zone/press (and much of this would apply to man/press as well) lies in its flexibility in late game/close game situations. If you're winning late, you can set your defense to drop the press half of the equation and go straight zone to reduce your chance of fouling and giving the opponent an opportunity to catch up at the line while the clock is stopped. If you've been playing 2-3 zone/press, you can even go straight 3-2 here and really guard the 3-pt. line. On the other hand, if you're losing late, you can drop the zone (or man), run straight press and rachet up the pressure a notch from the level that it has been at. If you're playing a fullcourt press team this will usually see you with a fatigue level advantage at the point you slap your press on.
Regarding IQ in a combo, I didn't see any problems bringing in kids with a B-/C+ in the press half. I wouldn't START those kids (unless their offense ability was just off the charts) , but I didn't panic about bringing them off the bench for spot roles.