Pretty Useless, ya?

In mose cases not.

I have like 1 million of them by now :D

I think time to time you can find some deals in shop where you can spend these vs blue or gold ones but that is rare. Also most recolors I guess can be purchases by these if you are the person that like to recolor the same skin into probably even more ugly color :D

I did buy something with these recently but I can't remember what was it = probably something totally unimportant and I was just bored of having 1 million of these laying around so decided to splurge on whatever it was :D

I personally am not interested in any skins or especially heirlooms so to me all of those have been redundant forever. I have just bought some skins of guns that have better iron-sight and some skins for legends that don't have black/grey skins to be less shiny / visible on the map. That's about it.
No other cosmetics used ever :)