I'm speculating heavily, but it might be beneficial to precompute some kind of timing map for a large portion of the network in order to have a baseline for future timing analysis attacks. I only have a very vague idea of how that might be useful, so this is low probability.
Ok, I can't think of an attack that could be accomplished by controlling half the traffic on the network. It doesn't mean there isn't one, but I can't think of it. I think that was a bad guess on my part.
Guard nodes make it so only a certain number of certain types of nodes are visible to any client. By controlling a huge number of clients, you are able to enumerate them much more easily.
