The relative protection has been discussed already, that it's better than nothing but not nearly as good as a hardshell helmet.
I raced in the 1970's and used a Kucharik helmet which was the best harinet style at the time (IMO). It had large front and rear padded sections, pads around the back of the ears (which greatly improved comfort) and less space between the pads on top. I "tested" it a couple times, lastly at the Marymoor velodrome in 1977 when Chris Springer "helped" me into the rail on turn 1. I proceded to skid/roll down the entire embankment from a considerable speed.
Many of the track riders wore Skid Lids which outwardly were a step up from a hairnet. The original Bell helmets were just coming out. I later heard that Skid Lids were outlawed because the openings were too large but I would have rather crashed on one of those than a classic Cinelli hairnet.
I trained many years back then with no helmet. These days, that just seems to be right next to stupidity but then I'm older/wiser.
Gary Watts
Vancouver, Washington
USA