TL;DR: IS VISUAL CUSTOMIZATION PART OF GAMEPLAY? DISCUSS
I strongly believe it is. Selling skins on launch is equal to selling other parts of the game.
This going to be an autistic wall of test, but bear with me for a while if you want my reasoning.
Do you think that skins, costumes and all other types of in-game cosmetics are not tied to gameplay? There is a statement that I hear from time to time, it goes kinda like this
Is that so? Major factor, primary even, is to have fun while playing videogame. "Fun" not as in "happy-happy joy-joy lets laugh like retards", but as in "feeling of content while performing tasks, overcoming challenges and indulging into activities game presents". There are plenty of serious and dark games that are no way happy or joyous to play through, but still perceived as "fun" because you feel content while playing them.
Here comes my idea of character's appearance factoring into enjoyment coming from video game. Remember backlash Diablo 3 received because of WoW aesthetic? Imagine if the last souls game or MGS was done in some cartoony, overtly bright style. While I absolutely love both western and eastern (anime) cartoon style, you can't argue that visual presentation fitting gameplay mechanics is universally beneficial for the game. If light platformer about jumping on enemies heads and collecting gems was done in turbo-realistic serious tone without any satire to it, shit would be just as difficult to stomach.
What I'm trying to say is that ability to customize your character's appearance is an essential part of enjoyment you receive from the game, and thus of gameplay.
Which means that selling character skins as DLC on launch is the same as selling chapters or levels or characters or any other gameplay content.
Being able to alter character's appearance in accordance with one's tastes is as big part of gameplay as using favorite type of weapon or adopting favorite strategy of beating challenges game presents.
Of course it depends on game in question. In games with set protagonist like Deus Ex or Planescape and sort of customization isn't needed, but EVEN THEN you could customize face of Denton, how cool is that?
But when we're talking about RPGs and, even moreso, online games, where character is stand in of player, an avatar representing you in game's world, being able to alter his or her appearance is primarily tied to your enjoyment and is inseparable part of core gameplay.
Thoughts?