Although it may seem strange to get excited about DLC, the possibilities with Premium Skins are almost endless. In Injustice, Premium Skins are essentially costume changes that also change character intros and character dialogue. This allows for Netherrealm to include almost any character they can think of without having to create an entire moveset for them to fit into - rather like how the Xenomorph in Mortal Kombat was a play on Baraka's base animations and moveset.
Why is this a good thing?
Well, it allows fan favourites like Reverse Flash and Jon Stewart to appear in the game without taking months of development, and without costing twice the price. Premium Skins are the cost effective way of allowing variations of characters without taking up a character slot, like in Super Smash Bros.
Knowing Netherrealm's generosity, fans will probably be given at least one free Premium Skin in the game's lifetime. Fingers crossed.