I would argue that paizo made it a GM book specifically because they intended it to not be made available to players without express GM approval - that it is intended mainly for NPCs and is to be limited to PCs for only very specific instances.
I wouldn't allow general use of the material in any and every campaign or every PC.
James Jacobs notes on this page:
"It's in the Campaign Setting line for a few reasons:
1) We wanted more room than a 32 paqe book could give us to cover all of the things we wanted to cover in the book.
2) We want to periodically do rules-heavy releases in the Campaign Setting line.
3) There's some stuff in there that not all GMs will want players to use, but that they'll likely need in order to run certain NPCs. The Campaign Setting line's pretty much our ONLY place to do this when topics aren't related to an Adventure Path. For example, we have an archetype in this book for winter witches—that's an archetype that most GMs won't want PCs to use, since it's a "bad guy" archetype, but we're not doing any Winter Witch AP anytime soon so we can't really wait to put this info into an Adventure Path volume. Putting it into a Player Companion would give the impression that Paizo is saying, "Hey players! Now you can be winter witches, and there's nothing your GM can do about it!"
Also, we HAVE done plenty of books in the Campaign Setting line that are appropriate for players... "Faction Guide" is hardly the first. Other examples include "Gods and Magic" and "Seekers of Secrets." And there'll be more in the future."