Well, that's why there is still a discussion. It could be argued quite strongly that Donovan has been the best MLS player of all time. Giovinco may eventually be seen as the best ever, but we have to wait for a little more track record. No, Giovinco would not have played in MLS 1.0 for sure, but Donovan did.
Henry is one of the all-time greats, but Donovan played in MLS in his prime, something Henry did not.