Yeah, I mean, any game can throw in bells and whistles such as achievements, trophies, ranks or progression systems but if those are the things keeping you playing I'd have to wonder if you enjoy the game at all or if it is just the systems you enjoy.
I've been sucked into terribly bad flash games purely for some challenge or something that keeps me playing. Maybe people can relate this to past Halo games but after each session of "the game that just isn't fun but has addictive features added on" I mostly feel guilt with a small sense of 'worth' that only lasts as long as it takes for me to realise that any challenge or trophy doesn't have any substantial worth except the time you wasted with it.
I'd have to argue that this is an overly-simplified way of looking at the situation. I've had (and still have) many gameplay experiences where the competitive nature or the leaderboard features are a huge part of why I play them so much. That doesn't mean that I wouldn't enjoy the game without them, but they are
another layer of enjoyment on top of the base game that also has the effect of extending replay value.
You talk about playing games simply for challenges. Why is it so bad that some people treat higher skill ranks as a challenge? I've gotten all acheivements and 100% on games like Super Meat Boy (106% in that case), Geometry Wars, Rock Band Blitz, Alan Wake, etc but I keep coming back to them to compete on leaderboards, because I find it
fun and challenging to try to beat my score or my friends as well as moving up on the leaderboard positions. That doesn't mean that I think the games aren't fun otherwise, because I enjoy the shit out of them. I just enjoyed the leaderboard/ranking systems as well. In H2/H3, it was fun and challenging (not as challenging in 3) to rank up, cheaters aside.
I don't understand the criticism of people who enjoy these parts of games (assuming they're not jerks about it) and want to know if a product they're using their hard-earned cash to buy has a feature that they consider valuable. In a consumer economy, I feel it is a valid concern.