A lot of what has been said above is obviously true. Clearly, Bootcamp will be faster than Parallels. But, the point is that Parallels can be fast enough to avoid the huge hassle of booting into Bootcamp, and is good enough for a huge number of games.
I played F1 2012, Skyrim and The Witcher 2 quite happily on Parallels 8 albeit with lower fps and some detail turned down.
Out of interest, my WEI results for the 5 categories on Bootcamp were 7.9, 7.9, 7.5, 7.5 and 5.9. On Parallels they were 5.8, 7.5, 7.1, 7.1 and 8.8.