I slip a lot, I always hit the numbers at home and I need to use a slip to do that because my judgement from abeam isn't that good, usually short.
Slips are useful for another reason not yet discussed. I slip at higher speeds so I keep my margin from stall and my clean configuration so I can abort and escape quickly and not near stall, not in flap range.
The mountains I frequent are really big and relatively speaking higher CT speeds are still quite slow. Shear is a big concern so I always keep my speed up. Often I need a steep descent so I slip it at a higher speed in a clean config.
tl;dr; slips are useful for steep descents at higher speeds above flap speeds