Timo,
the idea of a MySQL backend isn't because you want to win a speed contest against other storage methods. The idea is because with MySQL you can do things that would be nearly impossible without a database. There are things I'd like to do that are basically unthinkable now. Databases would allow me to do indexing on a whole new level. I could add awesome featues to my email system and it would be incredible for spam control.
On example for spam control. Suppose a spam attack comes in delivering the same spam to thousands of users. And suppose that my system didn't realise it was a spam until after the spam was delivered. I could delete all email from that host for all users in seconds if I make the sender's host address a key field. You can't block spam retroactively without a database.