I understand the need for flexibility, if you are on standby. But, given that rosters are published well in advance, you can take advantage of a firm seat on another carrier, for a price better than staff travel. All it takes is a little forward planning.
If the flight is cancelled or delayed, then that can happen to any airline, and there are EU rules governing the situation.
Since BA pulled off Spanish routes from LGW, I wonder what the crew who used staff travel are doing now - using EZY of course. OK you risk losing your money if your inbound flight is delayed, but at least 80 percent of the time you will be financially quids in, and not on standby.
ps For those LHR bound EZY is now the ONLY carrier from LGW on some extremely profitable Spanish routes, BCN for one!
Why pay over the odds for staff travel if you can plan ahead? It simply doesn't make sense.