this model is completely fine. as long as a payment model doesnt stop you from doing things then its fine. like in mass effect 3 and fifa 13. EA are on to something. I will never buy DLC myself but the options there if you want. As long as they keep it an option who cares.
It diminishes the accomplishment of other players, as well cheapens their own personal experience.
Not many people managed Dead Space 2 on Hardcore, if you beat it you know your were one of few ho got that trophy/achievement. On Dead Space 3 who cares because anyone can just buy their way to victory.
Uncharted 3 did something equally as stupid, you can now pay to unlock everything in the multiplayer, so where as you got the option to spend 30 hours farming treasures in certain game types, you can just unlock the reward for 99p.
Who is going to farm those treasures now? 30 hours of life vs 99p, hmm, tough one. Those 30 hours may have been worth it if you ended up as one of the few people with the unlocked reward to use, but when everyone can now just 99p unlock it, why bother?