The fact that a garment is undamaged after a crash is proof that it DID provide protection, not that it didn't.
It's not the same as a helmet, where it absorbs impact and is destroyed. Fabric protects by sliding. Different fabrics protect to different degrees. Skin-tight shiny lycra is the best for sliding against your skin. No thin fabric will protect against large, sharp rocks. Obviously it's not motorcycle leather, but it's some protection.
As far as tan lines... find a nude beach or move to the country.
Considering all the other lines (zero tan on the front of the torso; Y stripes on the jawline from the helmet; whatever type of gloves you wear; socks; darker on the tops of the thighs and the backs of the calves than anywhere else; etc. - sleeves hardly makes a difference.
Speed comes from what you put behind you. - Judi Ketteler