Advance Ticket restrictions are perfectly easy to understand and are very similar to aeroplanes - you can't go to the airport and demand to be on a different plane to the one on which you're booked without paying a lot more money. It's Off Peak restrictions that are complicated and often unfathomable. I use AP tickets as I need to save the money, but I do wish there were more reasonable walk-on fares available. Failing that, a ticket that was a bit more flexible than AP (valid on a choice of 3 trains or something) would be useful, but I imagine that would be open to still more abuse!
Anyway, there was an ad for Virgin Trains in the paper yesterday which encouraged me to "Beat the system with Virgin" so perhaps you reap what you sow!