https://en.bitcoin.it/wiki/P2Pool#Payout_logicA miner with the aim to harm others could withhold the block, thereby preventing anybody from getting paid.
Incorrect. The miner can still be paid for the shares he generated which were not a block solution, because another peer might generate the block solution after. Thus the share withholding miner can parasite income off the pool while robbing the pool of his major contribution to the revenue. So this brings revenue to the miner, while parasiting on the pool. If enough shares are from attackers, this destroys the economics of the honest miners in the pool and probably destroys the pool.
So if you owned a centralized pool, it would probably be in your interest to attack all the P2Pools.
Whereas centralized pools could (in a redesigned block chain) implement Meni Rosenfeld's oblivious shares to thwart the share withholding attack.
So as I wrote upthread, P2Pool is not a sustainable nor reliable solution.