PvP with skins an advantage?

Yes they will be. Costumes especially. Allegedly cosmetics will be tied to your gear type (light, heavy, etc), but that doesn't really help with pvp. And we'll have an icon in our target's nameplate that indicates what kind of gear their wearing, but unless that icon is always visible (w/o clicking on the target) I personally think that cosmetics will definitely influence pvp in a bad way. But alas we live in the age of cosmetics, so they're inevitable.

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Yeah, that's why I keep trying to get them to clarify 'the costume question™'. It's not big but it exists.
If you can obscure what type of gear even if only briefly with no limitation, it is still going to affect the first few interactions of combat. Depending on the match up, skill gap, and gear/lvl that could be a determining factor. Nameplate only goes so far to combat this.

I still have no idea how they plan on handling racial skins. They have never mentioned race being on the name plate and given waterfall stats it matters almost as much as hiding your gear type if not more.

Hopefully they never sell cosmetics that alter sound effects. That always gets spicy soft p2w in games that do have them.

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It's about picking out high value targets in that 40-player crowd. If I see a 40-man raid approaching our boss farm I know there's at least 5 healers there. Considering that Ashes lets you wear any type of gear, I'd prefer to know which of those healers I should attack first, and the gear would tell me that. If I'm a ranger I might want to attack a dude in robes or light gear, if I'm a mage - a heavy dude, if I'm a rogue - robe/light would most likely be the priority.

But if all I see is just dudes wearing the same costume and I have to click on each healer to know their gear type - my dps output and pvp value would diminish. Some people might say "that's a feature that adds mystery to the fight" and I'd say that mysterious pvp is not fun. I'd rather have both sides knowing who to kill first and then use your skill lvl to win that situation than everyone running around like headless chickens in hopes of outdpsing their opponent or just getting more lucky on their first strike.