no
I like the way Tripwire are doing it at the moment. Skins don't affect the balance in anyway. So nobody complains.
Now if they start to make unlockable skins it will cut down the DLC skin sales. And they will have to find other ways of making money. A weapon pack would bombard the forum with complaints since you get an advantage through money.
Like I said before I like the current way, and it's a way that encourages the designers to make better, cool skins.