Yes, K9s is not a great hand, but with low blinds in these kind of games, you need to mix it up and limp a number of hands trying to see a flop cheaply and get hit squarely by the flop and win a big pot. There are also plenty of pots that no one really wants that might be picked up with a small bluff. K9s can make second best flush, second best straight, and can easily flop top pair with either card, so not completely useless either. Of course if you miss the flop, you have no draws, and someone else shows aggression, then it is a quick fold.
An unpaired 2 COULD be in the opponents range, but most players will fold hands with an unpaired 2 unless it is a suited Ace hand, because making a pair of 2s can only give you 4th pair on the flop or river. Even on a paired flop with a 2, your pair of 2s is vulnerable to ANY card on the turn or river. Also, the fact that the flop produced a pair of 2s suggests that the deck was 2-rich, which slightly reduces the probability of a player in the blinds holding a 2.
By making a pot-sized bet on the flop, I hoped to finish off the hand, or at least eliminate most opponents.
Had the opponent had a 2 in his hand, he would probably have raised my pot-sized bet on the flop, or betted on the turn, because trip 2s is pretty vulnerable to any card giving opponent with a pocket pair a full house, and a limp from UTG is strongly suggestive of a small pair.
From what I had seen of the opponent in previous hands, he seemed to be making a lot of bluffs, or squarely hitting every paired flop, and and I thought that here he might have a smaller-than-nine pocket pair and be trying a semi bluff on the river. It did not seem very likely that he had called the flop bet with A3 and made his straight. His river bet seemed a bit big(pot sized) if he wanted to be called. Perhaps the possibility that I held 22 was also in the back of his mind, not that I really have any idea what was in his mind other than trying to convince me that he had a 2.
In fact he had called a pot-sized bet on the flop with nothing more than 2 overcards (which I partly suspected) and had he paired his King on the turn or river, might have lost his whole stack.
Obviously you have to take some chances early on in these games, and this one happened to pay off. I almost folded the river, but something seemed fishy, so I called.