I know this has been discussed previously, but one problem is how do you know who is part of a pool? I could set up 10 pools (10 servers) with 6% each and then they'd be under the threshold individually, but I'd control them all.
One solution that is out there is p2pool which helps avoid the centralization. (The other question is whether or not miners wouldn't just switch pools - it has happened in the past for various reasons).
Some interesting links:
http://en.wikipedia.org/wiki/Sybil_attackhttps://bitcointalk.org/index.php?topic=309629.0:-)