I've seen a few platformers that do not have any collectibles and it's the platforming itself which is challenging (my platformer itself is challenging), but I think adding collectibles will add extra challenge and re-playability, especially if some of the collectibles are really well hidden. Tradition would dictate that you should have collectibles, as it's a way for players to gain extra lives or even gain points, but that would not be the only reason to have collectibles.
I'm not interested in making features for feature sake. I'm purely wanting an opinion on whether collectibles should be included in a game and what should the collectibles do when you collect enough? Is a simple +1 life fine? Should there be some cool object you get? Possibly should collecting really change something hectic, like collect 100 coins and your character is invincible?
How do I decide if my platformer should have collectibles?
Closed. While I disagree with the closing as according to the FAQ What should be asked
game design (level design, gameplay, mechanics, etc). I got some nice pros and cons from everyone. I have updated the question slightly to be more specific.