Judging by the article and comments, it looks as if people are living by a creed to be effectively altruistic.
In general any philosophy where you think there is an optimal answer suffers from the risk that the answer could be wrong.
In the case of givewell, one limitation is that you only support causes that demonstrate impact -- you don't support causes that take risks.
I'm sure there is many other reasons not think that everything can be solved through the application of logic :)