In the last update for Battlefield 3, some new pieces of DLC is available. They are the Shortcut Packs, that allows the players to unlock all weapons for certain kits (or all of them), by paying for the DLC code, instead of playing their way to unlock them. At first, I was a bid dismayed for this. It meant that people with enough money can just start the game with all the cool stuff, while the poor ones needs to work their ass to get them.
But them I remembered that in a game like BF3 the weapons you have doesn't matter, but the skill you have with them. Many times I killed opponents with way better levels than me because I was clever or just a better player than them. There is no super weapon in the game that allow players to be vastly superior than you, and usually when there is a weapon somewhat better, they just nerf it to be not an unfair advantage to it.
I think there is no problem in selling early access to weapons and other game items as long as other players can have access in-game and that this items don't give any unfair advantage to opposing players. For example, if it was a case of selling a car in a racing game, but this car being so much better than the free cars, that anyone with the DLC car cannot be beaten in a race except for people with the same car. In this case I am against this kind of DLC.
But I don't like the 'unlock within time' system too. It gives and advantage for people with more time to play than others. So, in a week, someone who can play all day all week will have unlocked way more than someone who can only play a few hours during weekends.
I prefer a system where everything are available from the beginning, you just need to spent in-game money you receive to buy them. So, if you are looking for that sniper rifle who looks so cool, you don't need to play for 50 hours just to finally have it. You buy it and start right away. If you want to test other rifles, you just need to play the game and keep buying them until you find the one you like most.
I don't know about others, but it does not make sense to me to play games just to unlock things. I know it is nice to do it, but I think a game must be more than hoarding items. It must be about the enjoyment of playing the game, not about how many virtual items you have.