I put together a sheet describing all the essays required for each school I was applying to including any optional essays. For my sanity, I wanted to keep the number of essays I wrote to under 4, and was hoping I could get by with 2. I narrowed it down to 3 distinct essays.
After writing the main PS, I looked at each school to see if there was a place where I could logically drop a "and this is part of why I want to apply to X" without making an major changes, but if there isn't a place where that fits with my PS as written, I'm not gonna sweat it and just leave it generic.
I have two optionals, one of which will apply to several schools. I left the opening paragraph and concluding sentence available for tweakage for that one to make sure that it was completely clear as to how this topic applies to the stimulous. In some ways, I think answering an optional essay in a completely on-topic way is more "personalization" than you can possibly do with a "I want to go here because" paragraph.