Collectables that earn you unlockables or are necessary for 100% completion aren't a problem; they're there for the dedicated gamer or the obsessive completist, and they add longevity to the game. Mandatory collectibles are another matter, particularly if a large portion of the game is dedicated to merely acquiring these otherwise useless items. I don't like the idea of being unable to progress in a game, not through a lack of skill, but a lack of free time. The GTA series is a well-known example of how to do collectibles right. They aren't necessary for completion of the main storyline, but they offer rewards to those committed players who are willing to seek them out.