How thick is this type of line? I find fishing line to be a lot thinner than something like #18 cave line. It will definitely be strong and less likely to snap but being so thin it wouldn't be good to attach to things like rubber hoses. I feel it would cut into the rubber.I use 200lb test braided downrigger line. 300ft costs about $30 and will last you forever.
Additionally, the way I attach boltsnaps, when the cave line frays I can see it starting to fail before it completely snaps. I inspect my attachment after every dive. If I see any fraying, I cut it off and reattach the bolt snap with new line. With downrigger line, don't you risk the chance it will snap suddenly and you could potentially lose your equipment?
What is it made out of? The nylon of cave line responds well to melting with a lighter. So I can ensure heating the ends and the knot then pressing them together will fuse everything and prevent it from undoing.
Bottom line, cave line will need to be replaced more often but it degrades much slower. So I can see when it is starting to fail and replace it.
I will admit, downrigger line probably handles salt water better but I dive mostly fresh water.