But Willi, it happens all the time that someone starts a long-document project in Word which must then be transferred to InDesign at some later point. You're exactly right when it comes to "best practices," but what do you do when you're not in the best situation?
The cleanest InDesign file in a workflow like this one, I'll be happy to admit, is one in which all Word formatting is stripped at import. Sometimes a Word file is so bad that I save out raw text from Word and place that, re-applying formatting as I go. On the other hand, it happens all the time that I create styles in InDesign, export RTF, let my translators work in Word on those RTFs, and then re-import the translated RTF files, stripping Word garbage out at the import stage.
But Michael's question was obviously not about InDesign best practices. He's trying to figure out if it is worth his time to fix the Word garbage in InDesign. Espcially in the case of losing nonbreaking hyphens and spaces, it's a terrible idea to throw away all Word formatting and start fresh in ID. Careful preparation in Word followed by careful import - which will allow you to toss some garbage formatting and keep some, er, non-garbage reusable stuff that originated in Word - is, I think, a perfectly valid workflow.