I have updated one (actually older) Macbook Pro to El Capitan and FCP7 works fine. I have even run a recent project on the system for a few hours and all seems to be well. (I did not use the installer, but rather already had FCP7 on the computer — so I can’t speak for the installer issue that some people are reporting)
As far as Yosemite, I’ve been running FCP7 on that for months and had not had any issues at all. Works great on two different systems.
As far as the issue of why still use FCP7? … As a few people have already noted, we all have our reasons. Just leave it at that. Some of us STILL like it very much.
I, for one, still edit all my projects on FCP7, and then export the final XLM into FCPX (7toX) and finish the titles and color there. (The audio is a mix between FCP7 (OMF export) and and FCPX — don’t ask, but it works for me) FCPX is quite nice for many “finishing” things but I would hate to edit an hour+ documentary on it. It’s just not really great for long-form projects.
The bottom line is this, FCP7 is still great. So, not to be a hard-ass, but if someone posts a question, shouldn’t we try to answer it rather than tell the person to forget it — It’s really not helpful. We’re editors … we solve problems. That’s what the job is, because God knows, they didn’t do it on set.