Obama: Went back on his word, yep.

He went back on his word to meet with the Republican nominee and set rules for public financing. Yes he did. He didn't outright say he'd take public financing regardless of anything. Obama's campaign is also involved in a bit of spin on this though. Although a majority of his donors are $100 or less, the majority of his money is still from big donors. That'll only increase as some of Hillary's donors help out. Yes, this situation is that simple.
It isn't likely that whatever minor hit he may take for opting out will negate his spending advantage. Yes, it does add to the attempts to paint him as untrustworthy with some voters but again, the spending advantage because of opting out should off-set the hit.