Because he has, or develops, any one of uncountable delusions of how a majority pool can be used. (or someone related to him/can access the pool/can pop his crap)
As I posted over on another thread( dealing with this topic only 15% of people (or less)) get....
The road to hell is paved with good intentions.
Trust is everything in this game, unfortunately most of the players are unfamiliar with that concept, and freely give in to nonsense (in RL, see tea party voters).
...accidents/mistakes/someone finding where he lives/someone ddos another pool/random craziness happens.
I'm really not so sure. People don't really have any reason to pledge allegiance to any particular pool long term, nor is there the real life problem of making a choice (voting) and then being stuck with it for years.
I mine with BTCGuild because I like their website. A day from now I could decide I like someone else and mine in their pool. A day later I could switch back. There is hardly any cost in doing so, for me or any other miner, so any power a pool operator has is extremely temporary and entirely contingent on keeping their miners happy. That makes a pool operator doing bad things seem at least fairly unlikely.