> these days is there any good reason to back up the rest of your system

From experience, there is definite need. I spilled some water on my work laptop and it died. I was able to get a replacement in maybe 3 hours, but setting everything up again was a major pain.

A Time Machine backup would have let me continue more or less where I left off in a matter of an hour or two, vs. many hours/days (and some lost work). (not that Time Machine is perfect either, but much better than just iCloud)

I agree that getting rid of system cruft can be good, but it's better handled proactively than on machine failure IMO.