It works well for 99% of the time, but when it doesn't, this design decisions make the experience frustrating. I'm-going-to-write-a-blog-post-and-boycott-this-thing frustrating.

And if the description is not clear (as in the Rdio example), it'll surprise the user. And surprise is not good. Let's say you are mindlessly managing your collection, doing some syncs along the way. When you finish, you notice that there are musics in the collection that you didn't put there. Even if that was exactly what you wanted, you don't know how you did it and now the software is not predictable anymore. You'll flinch before using it again. I know I would.

Multi-action options are a bless when aligned with your flow, but if not used carefully they'll bring down the whole user experience.

I think engineers know about use-case analysis. Don't bother to present the user with options for which there is no use-case. This isn't some sort of voodoo that only "product people" understand, it's software engineering 101.