The hash rate itself is irrelevant, what matters is the cost of achieving it. If that cost becomes too low so that someone can afford to control a substantial fraction of the hash rate, they could decide to mess with the system, for example perform a denial of service attack. As said before, it seems that running Bitcoin currently costs about a billion dollar per month. That makes the network probably quite safe but it certainly is still in reach of some actors. Lowering the costs substantially - say ten times or more - would increase the circle of entities capable of messing with the systems quite a bit.
The question is of course why someone would want to do this, but I can imagine some scenarios, not at last that it seems quite possible to make some money if Bitcoin experiences issues.