Yes that does happen. See what happens is, all the people involved in a pool chip in and buy what are called "pool sharks." The shark entities patrol internally until they are able to jump a competing pool. Then, the two groups of sharks fight, and the winner can usually grab a few bitcoins to take back to his pool before another shark is generated or the pool is economically "poisoned" to all sharks, enemy or ally.
However, deepbit moved on from sharks to A.P.E. programs, which are better for long-polling combat in the areas outside of pools. It takes longer for competing apes to find each other, but combat is more effective. Apes can jump into enemy pools to attack sharks, but not the inverse. However, apes cannot steal enemy bitcoins, with a few rare exceptions.
Where it gets really complicated is hash slinging. A pool can take some of their hashpower and direct it to another pool, thus confusing their miners. When your miner says "Result: hash rejected" it means that someone slung hashes into your work queue and you were fooled into doing worthless work. Right now, because I'm near the edge of the pool and thus get attacked by enemy hashes constantly, I have my own pool shark that is entirely supported by slung hashes, so it doesn't cost me anything until an ape attacks it. The evolutionary arms race has stepped up, recently, where brown hashes are slung instead of the typical rainbow tabled ones. Both are hash combinations, but hashbrowns are computationally "delicious" to sharks, which may be fooled into temporarily changing sides.
Entire papers have been written on the subject of bitcoin pool combat, so I won't go into the finer points except to say that the oneupmanship is progressing so quickly that, really, only the pool chiefs know the current state of interpool war technology, and they are dark, mysterious entities who we aren't allowed to name.
Post your goddamn bitcoin address. You win this thread.
- vandalizing competitors has a very large disincentive if caught which far outweighs the incentive. But the OP asked if an incentive existed, and the answer is yes, there is that "incentive". 
