I once worked for a company where I heard this saying a lot. I never agreed with it, and I would have to guess that most smart people are in total disagreement. Reading blogs earlier I came across evidence to prove, at least creatively, that one head is better than 2.
The reason for this may be a little hard to pin down, but for starters I would say that A) In a group some people are not going to speak up and B) any great idea will get marginalized by a committee.
So why do committees still exist when thy have proven to be less effective? A committee has never gotten fired.
BTW the ugly stock photo is on purpose – it has that great committee feel, no? And BTW2 that company I used to work for has struggled a great deal since I left.