If they've added Zaku, it won't be long until they add Dosu and Kin too. They will eventually run out of characters so I bet they'll be added. I'd say they're trying to stretch it now that there are so many of them out, so they always have a new release from time to time. I don't see the point of releasing Zaku on its own, but that might be the reason. Although, story wise, they should have already been added, since they're from the early chapters of the story and were one of the first villains that the genins fought on their own. There are a lot of characters that only appear in half an episode during the GNW, or filler lol, like most of those seven swordsmen of Kirigakure. I kinda agree with the Mabui thing, and I'm sure when Rin is added, she'll have some decent taijutsu chase, even when she couldnt even avoid getting kidnapped. Twice. Mind you, I dont want Rin but I guess eventually they'll add her too.