So, if one is already a Werewolf before buying Dawnguard are you weakening yourself by missing out on perks? Or does it simply start leveling up by how many people you kill while wolfed-out? I'm just curious because I've recently got back into Skyrim and didn't know if it would be worth it to go ahead and buy Dawnguard before I would be playing the content, just for the perk changes in Werewolf and Vampire forms.

I was a werewolf but never really used it that much. I played the Dawnguard side, and the werewolf part did not stop anything. You do have to have the Totems of Hircine in order to use the new totems on the perk tree.