Some ISPs (I don't know if it's accurate to say most) have last-mile or middle-mile equipment that does not support IPv6 and they can't afford to upgrade it, hence kludges like 6rd. Of course good ISPs don't use kludges, but then you're getting into no true Scotsman territory.
This is the experience I had with CenturyLink, they support IPv6 via 6rd and it sucks. It's annoying I don't have native IPv6 support with CableOne either (and they're too cheap to bother upgrading anything beyond the DOCSIS head-ends to charge you more money), but I'd rather use a Hurricane Electric tunnel if my ISP can't give me native v6 in the first place.
