I believe there should be a new tab in the shop called Achievements. Now, you wouldn't be "spending your points", but at a certain point, you unlock the ability to buy certain items. Much like how you level up and are able to build the new buildings, troops, etc.. Having it unlock the ability to buy items, rather than spending your points that you can never get back, it allows for items to be added/removed without having a lot of people mad, because they spent their points X days before item Z is added to the shop.
Now, in the shop, people would have to buy the items, whether it be for coins or gems...
It would give people a reason to do achievements other than for e-peen contests or OCD :P