Re: "What's wrong with a /64 prefix?"
Compared to the status quo of v4, sure, a single /64 is better than no v6. But we can aim a little higher than that. There are things you can't do with a single /64 but you can do with a /56, which is why you shouldn't be getting a single /64.
(Or to be more precise, it's why a single /64 shouldn't be the max you can get. If a user only requests one /64 then go ahead and only give them that much, but they should be able to get more without hassle if they want to.)