So tell me, how often do you have to inherit from another class at a later time?

Fairly often - that's why I said it ;-)

I find that I add MI (usually only temporarily) during refactoring. A class pops out from another class, becomes an inherited subclass for a bit, and usually ends up in a has-a/implemented-with relationship in the end. So I can start out with single inheritence, move to multiple, and come back to single again.

Using qw() consistantly makes things easier for me.