For single player games, not at all. Though any major multiplayer game like Halo or COD, yes install base is very important.
Edit: Nvm. I misread this. I thought you were implying the amount of people playing a game. Yes, it does matter. But even with a decent install base, 3rd part Wii U games for the most part selling next to nothing.
Made a bet with LipeJJ and HylianYoshi that the XB1 will reach 30 million before Wii U reaches 15 million. Loser has to get avatar picked by winner for 6 months (or if I lose, either 6 months avatar control for both Lipe and Hylian, or my patrick avatar comes back forever).







