[Xylakant]

I agree with you that it might be nice if you could actually rely on it, but OTOH I have never personally encountered an error caused by a non-boolean predicate.

However, it's not only nonzero? or defined? that don't return boolean values. see http://news.ycombinator.com/item?id=5074676 for more examples. If you read through the core libs documentation you'll find more examples. You just cannot rely on predicates returning true/false in all cases, so you either have to learn not to rely on it at all or learn every example where it doesn't. So just don't rely on it.