If by FP+ you could be shut out of getting a FastPass to A&E 60 days in advance, then by FP-, you could be shut out of getting a FastPass to A&E by not getting there first thing in the morning. (We all know if SDMT was under FP-, the tickets would be gone in the first hour.)
I didn't see that in my experience. We rode as much under FP+ as FP-. The lines were not particularly longer. Figment did have a 20 min wait early in the day, but we Fast Passed it... so it was actually shorter. Then later in the day there was no wait.
There are varying reasons to install / not install it at other parks. The biggest difference is that WDW caters to vacationers who plan to visit months in advance, but
Disneyland is predominantly a locals park and for ppl who visit LA and do DL for a day or two. If FP+ goes to Disneyland, it may have some different functionality to accommodate their guests.