The whole "can't use existing accounts for play on Steam" is ridiculous in the first place. I have stronger words, but I wanna be civil.
That's completely false. Steam is easy game-management, too. I want my games all in the same place, I want developer notes and community stuff available with two quick clicks. I want my game integrated into the client and a "non-Steam" game like the original client does not have a lot of the functions I'd need it to have to be satisfied with it even if put into Steam the one could do before.