Proxy = single point of failure.

Proxys of other pool clusters seem like a good target to DDoS to me...

Also connecting to that proxy with as many (forged/fake) clients as possible and just dropping block announcements silently might also work (and be a bit more subtle) - should also highly increase your pool's chances of winning a "block's race".

Thirdly, non-proportional pools all don't need this Voodoo (so they would be far better connected to the network --> better chances at block solves!), and there are not that many "groups of 3 and 4" pools left for such stuff... thanks to the hoppers!