I have a connection to a rock gym and may be able to get my hands on some free climbing rope. The gym replaces their lines every 3 months and simply throws the old ones away. Would recycled climbing rope be practical on a cruiser?
Like others have said, no.
There are a couple of exceptions, but one's related to going up the mast using jumars or a similar climbing aid to save you halyards from wear and tear. (We've got an old 10mm climbing rope just for this purpose, but I know it's use history.)
The other I've read about is use as an anchor snubber. There's a photo and explanation in Beth Leonard's book.
the only other use I've wondered about for a smaller boat would be as a preventer.
Sheets, halyards, no. Tow line or dock lines for anything larger than a small light boat or dingy, No.