I hate some of the gimmick PPV's (Fatal Fourway, Money in the Bank, Hell in a Cell, and TLC). I like gimmick matches to be announced as a way to enhance, or end, a feud. When it's expected, it takes away from the angle, at least in my view.
Others serve a purpose. I don't like the Extreme Rules name, but it usually settles all problems from Mania, in some sort of brutal fashion. I would like all problems from Mania to be solved at Mania, but whatever. Night of Champions isn't really a gimmick, seeing it doesn't follow a Champion vs. Champion format (WHC vs. WWE champion, etc.). Elimination Chamber works, because it usually has quite the impact on the main events at WrestleMania. Bragging Rights, Team Raw vs. Team Smackdown, should probably just take place at Survivor Series.
However, I understand why they started the gimmick PPV's. Buyrates, plain and simple. They were hoping this would energize fans to purchase more off-brand PPV's (not Mania, Summerslam, etc.). Has it worked? I don't know, I haven't looked at the numbers. If it's working, they should keep them, no matter how I feel about it. If it's not increasing buyrates, or if they are falling, I would like to see them phased out.