I recently installed my first SSD in anticipation for ARR as well. The OS, programs, and games go on it, and everything else (downloads, etc) goes onto a normal HDD. Having an MMO installed on the SSD won't help kill it any more than if you install an offline game. It will get updated every once in a while of course, but so do other games like Skyrim.

Now, I wouldn't fill up the SSD with 100 GB of data, move it to a HDD, and repeat the process over and over and over. But I would hope that goes without saying.