I'd say it's more of an efficiency thing...

Look at the Wikipedia page on "law of large numbers" to see why it benefits miners.

The fact that GPUs do so well with the blake-256 algo does make it more difficult to attack the network with botnets.

Think about it.  You get gh/s on a gpu...  That's a lot of hashpower for very little electricity. 

A tip for miners:  blake-256 isn't very memory intensive, so under clock your gpu memory to improve mining efficiency.