Remember that the pledge is not to give money to GWWC; it's a pledge to give to effective charities in general.

It's not; the whole message of GWWC is about the strong reasons we in the relatively wealthy west have to give significant portions of our income to cost-effective global poverty charities. I completely respect those who think we have even stronger reasons to donate to cost-effective charities focused on causes like animal welfare or x-risk, but GWWC is focused on global poverty (which does earn it more mainstream credibility than, say, EAA or SingInst).