Again, why Gametap gets compared to Steam makes no sense. Gametap is NOT a digital distribution service. It's a subscription service that allow you access to full versions of games indefinetly until you cancel the subscription. Yes, it doesn't support mods but it's not targeting the hardcore gamer (reason it gets no love on this website), it's targeting the casual player that has no interest in mods, just simply wants to play the game.
If you want a game and knowing you are going to mod it later, buy it from steam/retail/etc.