Diamond slots are for people who want to give more support to the server because they like it (and can afford it).
Changing diamond to multiple months would kind of defeat the purpose of having multiple ranks, since they are all monthly...
And I don't see why people think it's a good idea to have 'extra slots' for donors. There are only 24 starting platforms in the normal servers to begin with, unless you want every game to have a max of 22 players. And then theres the fact that there are a lot more donors than two slots per server would allow, so eventually some people would get left out. Either way, non-donors aren't going to notice any benefit so why does it even matter?