I can make my web site/app extremely hard to use in all sorts of bad ways, the developer being able to disable autocomplete should be the least of anyones worries.

The other side is the situation we have now, autocomplete doing the wrong thing all over the place with no way to stop it. Stomping on my apps specific database driven autocomplete really hurts the user experience. Also autofilling fields without the user noticing and entering wrong data into forms. What a mess.