I wonder how many Event tickets are being used that way, though.
This is anecdotal evidence, so take it with a grain of salt. But in 19 years of running events at Gen Con, our group has had one player use an event ticket this way. It's still a pretty well kept secret as policies go. I'd wager at least half the GMs and most attendees don't even know it's an option.
I guess I'd have to agree with this since I've been going to Gen Con for 32 years and didn't even know this was a thing you could do. I probably wouldn't do it anyway, since I'd rather return the ticket so hopefully someone else can take the spot.
One thing I've wondered about is what percentage of event ticket sales are pre-sold (versus sold at the Con)? What I'm getting at --- if you preregister for an event ticket and then return it at the Con, how likely is it that they will find a buyer for it? Personally, I make a schedule during wishlist time, but seldom try to track down additional events once I am at Gen Con.