Even if you take the Friendship is Magic thing literally, Twilight is Still the best manager type Ponyville has. Just see Winter Wrap up for proof of that. She even managed to get them to finish on time regardless of the amount they were late by when twilight took the helms. She has a clear talent for both Magic/Friendship AND management. One just comes up more then the other.

And I recall with Rarity it stated her talent was for finding Gems, but lets say I'm wrong and that was never stated, and it is in fact for making Fashion with Gems is her talent. She's made Jewel-less clothes before. That makes no sense what so ever. Look at all of the dresses she made in 'Suited for Success' and you'll see the shear lack of Gems those dresses possess. These are designs that impress whatever that dudes name was who watched the fashion shows. So what you are telling me is she has a talent for using gems in her desings, yet is recognized for her jewelless outfits?

And yeah, your AJ explanation is a pretty damn big stretch.

The Cutie Marks work fine as giving you a glimpse into the pony characters, but the route they go with the CMC is just silly. Especially on the first episode with them where there is the whole thing about 'you already have your cutie mark, where as I'm a blank slate who's talent could be anything'. That is just a silly line, as apperently the moment they each get their cutie marks they'll be in the same boat. Which is really odd, as they treated that as though it were a bad thing.

And on to Rarity again, if Cutie marks signify a talent for something, why the hell hasn't she a famous designer already? Clearly she has talent, I mean look at her Cutie Mark. Is it because she still needs to practice to improve her skills? Clearly this is the reason. Now the question is can they improve things not related to their talent? Well, FlutterShy has shown herself to be a mighty fine Chiropractor, there is Twilight and her Managment skills, Applejack and her athletic talent, Rarity and her Gem Finding/making non-jewel garments/shop owner, so yes they can improve things unrelated to a cutie mark. Which really means a cutie mark doesn't really dictate what it is your good at.

That means cutie marks are one of two things. 1) A story adults tell children just to make them work towards something, or 2) a superstition the entirety of Equestria believes.

I think a cutie mark works much better as a glimpse into a their Personality and what not rather then their special talent.