[hombre_fatal]

Unless I'm mistaken, they're not talking about some cloud/streaming service, just exposing their music folder to iTunes.app so that iTunes can give them a GUI over it (like search/sorting).

Without the checkbox, iTunes employs its own local folder organization strategy and will update id3 tags and album art from fingerprint lookups.

[nobleach]

This is correct. For quite awhile after I ticked that box, and "lost" everything. (I still had all the files, they were just put together in an awkward way. I could use iTunes to create playlists that mimicked my preferences... but that took a lot of time) Once I started using Spotify, it just made sense. Yes there's a ton of overlap in music that I own (some I don't "own" as I had snagged it from the napsters of the day) and what I listen to on Spotify. But, there's also quite a bit more that I never owned... or is brand new to me.