If I could see the others hole cards i would be rich, rich, rich. The problem is you don't see the others hole cards so we have to make decisions based on the available information. The information we have here is one person raised. Could he have raised with AK, AQ, AJ? Could he have raised with 88 or 77? Sure he could have. 99 is favored over all of these hands. Even if you suspect villan might have an over pair this is still ok. If you do flop a set you can win a big pot against an overpair since sets are hard to put someone on. Sets are not as rare as one may thing. You flop a set about one out of every 7 times.
There are of course different ways to play any poker game
and I am certainly not saying that what I said is the only way to play. It just seems to me that folding a medium pocket pair to one raiser (even with others to act after you) combined with only seeing the flop 7% of the time when not in the blinds seems very, very tight.
I am not advocating playing real loose and playing any 2 cards by any means. Just don't be too quick to thorw away playable hands. Unlike no-limit, on of the keys to limit is post flop play and knowing when to get rid of a hand or when to call down or raise etc. For instance if you have 99 and the flop comes A,K,6, then you can safely toss your 99 to pressure, but if it comes A,K,9, now you can make a lot of money which is one of the other keys to limit, which is to try to make a big pot. With the same flop if you have AA the chance that you will win a big pot id lessened significantly for obvious reasons.
Anyway, sorry for rambling, Different opinions and debate is good. It's how we learn.