Mm… When it comes to games where the player buy-in is limited (in terms of “all the rules are here in this book”), I’ve seen players buy core books. They’re generally players who would at least consider running it, so it’s not a pure example, but I do think there’s some utility to having it available separately.

I’m not sure there’s much if any utility to having it available separately on the kickstarter, though. It seems like if you’re backing, you’re either running or at least committed. Probably “Here’s an (extra) print player’s book” should be an add-on deal rather than a backing level, since I’m imagining most of the folks who might want that are thinking “I’d like a loaner/table copy” or else “I’m good with tablet+PDFs, but I want the player book in print to have at the table.”