Actually several major MMOs do.
"But MySQL…" is light weight and efficient. Our game code actually includes a wrapper that transparently lets us switch between MySQL, PostgreSQL and Oracle. And our usage patterns lend themselves very well to MySQL — its mostly just a way to externalize data from within the game. It's only used as a back end.
And like any application, we generally avoid direct database accesses when possible – we don't save every change as it occurs, we use a lot of batching techniques. We also avoid direct database transactions where possible: we used to rely on a practice that has become fairly common amongst MMOs, a "proxy" – our own application that speaks to the game in game protocols that it converts into SQL statements – things like saving all the changes to spawn lists don't generate more than 8 queries a second.
The other technique being the use of worker threads that handle aggregated SQL queries maildropping the results to the neccessary callbacks.
There's also a little more magic behind it than that but I'll save that for the con :)
S! to the kgwikings :)