>  I've never understood the weird fascination with "many-worlds".

In addition to the parsimony/redundancy argument already brought up, our weird fascination with "many worlds" should be utterly obvious from a psychological perspective. It's a powerful, potentially soothing thought: anything that could have been has been, in some strange way.

Thinking about the "what if" constitutes a deeply human tendency (or weakness, if you will). The notion that all our missed chances, fantasies, dreams, ideas, etc. may be actual as opposed to potential -- that's way too attractive a thought to ignore.