While running for President, former US Senator Paul Tsongas famously said, “That's a good question. Let me try to evade you.” He didn't make it past the primaries.
There was a lot of discussion about WAF evasion techniques at this year's Black Hat. Imperva's Tal Be'ery, in his weekly Security Week column, gives an interesting take on the issue. In a nutshell:
By using evasion techniques hackers break the most basic principle of hiding as stated by Sherlock Holmes: “the best place to hide something is where everyone can see it.” The evasion technique usage just draws more attention from the WAF and actually helps the WAF to block the attack.