I just completed Tomb Raider and got all of the collectables. I immediately became very upset to know that all you earn for your troubles is just concept art and character models. Concept art (or fan art in other cases) has been my most hated unlockables. It's just a lazy excuse for the developer to use instead giving you something of real value. It's like the old 8-bit games with shitty endings, a black screen with text is all you get at the end - all this build up for nothing.
The game that's on my top ten list of games with the most shitty unlockables is the Dreamcast version Sonic Adventure were you earn absolutely nothing for collecting all the emblems.