They have reputedly released roughly 1% of what they received. I think that simply dumping the docs would have had less impact than the steady stream of new revelations, but certainly I could be mistaken...
1) Is it better to vet the documents, and remove personal information etc. where appropriate? If yes, then I'd rather they were released as they were prepared.
2) Would we still be discussing this if they had been dumped in one lot? This is maybe much more clever than you think. More importantly; I expect it's more likely these events will eventually seep into broader public consciousness if they don't come and go in a single news cycle.
If he did not have an expectation that they would follow his timeline, he would be less likely to leak the docs to them, and possibly less likely to leak the docs at all. It's not like Snowden couldn't have just dumped all the docs on us if that was his intent.
And don't unwarrantedly put people in harms way, &c. Yes. It's called journalism. It's not perfect, but neither would an unfiltered dump have been. And it's far better than us not getting the docs at all, isn't it? Which is certainly another timeline Mr. Snowden could have chosen, at far greater personal convenience.