Let's save the debate on whether achievements should be useful for another day. For now, let's just imagine that some achievements are useful, some are useless, and some would be useful if we had gotten them before we'd done all the stuff needed to get them.
To begin: faction discounts in World of Warcraft. Useful, right? I mean, they save on repairs and buying stuff. Here's the problem: by the time you're getting a major discount, you've probably done so many dailies, randoms, or whatever else that you're so flush with gold that you don't have much use for a small discount from occasionally repairing and less often buying anything, from that faction.
And while we're at it, they aren't quite achievements, but you've gotten exalted with that faction so you can buy the great rep gear from them. Awesome, except by now you've gotten better items from the heroics you've been running, with the badges you've been collecting, or used your generous daily gold to buy some BoEs. Admittedly this wasn't much of an issue in early Cataclysm, but we all still remember LK gear/badge inflation and I've not seen anything to indicate that it won't happen again eventually.
Stalker CoP loves these. Most of the achievements are going to be fairly late in the game, at which point you should be loaded with rubles. The achievements give things like discounts, better prices when selling, and more discounts. To make it even more ridiculous, the one biggest money sink, the guy who sells awesome equipment, never has this discount, but of course by the time you're getting these achievements you've probably either bought all his stuff or have proven that you don't need his stuff. And to top it off the final zone has a guy who repairs all your stuff for free, just in time for you to have gotten the biggest possible discount (beside free) from the other two repair people, making their rewards pointless as well.
Reward seems to be the issue here. Are they meant to reward us or to help us? This is one of the ongoing problems with gear in WoW.
On one side was the badge, rep, and crafting system which set up gear as a tool. We'd get gear to be able to raid. With the new philosophy in LK of always seeing the latest raid (even if none before it), it made sense that gear was steadily inflated, at least on the badge and crafting sides, but heroic and rep rewards just became obsolete. It even made sense that individual bosses would drop gear. This created a sort of difficulty slider, albeit an inconvenient one, whereby players who couldn't down boss X could kill the ones before boss X for gear to make boss X easier. And then even killing the Lich King makes sense for gear, since there is still hardmode, and of course those all make sense, up until hardmode Lich King gives gear, at which point we ask, why?
At that point the gear is clearly not needed by any stretch of the imagination and is purely a reward. It's not even vanilla WoW when the final loot of the final tier had an important role to play in helping to utterly wreck people in blues in BGs. Fun times...
And finally, there was an old fun game called Escape Velocity: Nova, the third in a line of games which are more or less single-player EVE with the combat system of Asteroids. At the very end you've saved the universe, taken on the greatest challenges, possibly conquered the universe while you were at it, and then you're rewarded with some sort of ridiculously awesome ship. Or in one case, you become such a powerful telepath that you can split off portions of your mind, three parts, and single part of which can go toe to toe with all but the biggest enemy ships. You're one step short of being a god. So uh, now what? There isn't a Game Over screen. You do get sent to a special magical planet where you get a nice "you're a hero!" message, but then you're right back in the galaxy. So, want to use your massive living ship with technology a hundred steps beyond anything else to... haul some cargo?
It's like the end of the game is the worst April Fool's joke ever.
RIFT: Khar! KHAAAAAAR!
11 hours ago