Erm, they're not doing it because it's fun, they're doing it because it pleases their sponsors. I am sure most pro-riders don't really like this stuff. It's a bit of a circus act.You're still comparing it to road cycling. It's not road cycling. And it doesn't defeat the purpose of having them to a physical effort in order to, you know, keep fit. They're just... having fun while they're doing it, and since there's nothing else to watch, it's a good way to pass the time.