because ppl like to see the net so they can judge better where to hit and because i think it might be a little dangerous..in case you fall for whatever reasons into the net...i think fishingline is not very comfortable to fall on and it can be dangerous/uncomfortable for the fingers when you build up and build down the net for every training session
OhSearsTower is right in that a visible net does help with the whole depth perception thing. Have fooled around with just a yellow rope as a net and it is very off putting. Does make one wonder how squash players learn to cope with those clear walls.
Have you ever played with an invisible net? It's really really bad, as you lose a lot of your spatial awareness. You absolutely need to see the net.
I'd prefer to see the tape on the top of the net clearly and be able to see my opponent more clearly.
Approaching the age where I'm about to need bifocals, I already have trouble sometimes with the net area, especially when I'm close up...