I've got an Olympic Arms preban lower that has a kind of gritty trigger and was thinking of switching to a NM trigger, but am not sure since this isn't goint to be a match gun.

I've never shot with a national match trigger, but I've heard that they can improve your accuracy quite a bit.

But I wonder if they are combat worthy? I mean, I know they could help with long range shots, but would they be too soft for rapid fire with a semi-auto rifle?

Like I said, I've never dealt with them so I'm really blind on the subject.

Any opinions?  

Match triggers improve accuracy because you'll find that it takes less effort tosqueeze the trigger, and the trigger won't have to be moved to the rear as much.
There are dual and single stage triggers. Dual stage does not move fast at all- useless for combat. Right now on your Oly is a single stage trigger with a pull weight probably around 8 pounds. You could get a match trigger so you could lower the pull weight, but these triggers are much more complicated, therefore may not be as reliable.

I've got a Jewell trigger on mine, and it's really excellent, but I've got a target gun.  In a combat weapon what you're interested in is reliability first and foremost.  The Jewell is considerably more complicated than a standard military trigger.  

Still, I'd rather have the Jewell, even on a combat gun.  I don't think it's all that delicate, and the improvement in trigger pull is worth it to me.  Even when it's adjusted fairly heavy it's no where near as long or as gritty as the mil-spec triggers I've tried.