There's a couple reasons I say this, playing since 2006 I'm still not used to having to search my events for a service of some kind. Not only this is the fact that if we are searching for a rush it's easier to just make a post and get the rush. Using my events you have to go click 6 different search options to find what you need. And on USwest nobody really bothers posting up an event since nobody will search for them.
There are a lot of successfull transactions for My Events I would agree 100% but there would be a lot more done if you let people post there in search of or there service provided. At least for Diablo, my suggestion is you can still use the My Events system for every other game but for Diablo I would allow people to post. It's just a hassle overall.
Like i said there maybe over 500,000 transactions have been done via My Events but I would bet there would be a lot more sevices provided (at least for diablo) if you allowed and didn't lock the posts.