The problem with scaling skills is that in a level roulette scenario where this type of level scaling would apply to high level players, you're still going to have the new player who is simply too low level to have as many skills unlocked.

While you may be level 70 and have access to 20+ abilities, that level 15 person is still only going to have access to 5. Unless you scale the potency of your skills to 0, no amount of level scaling is going to make your damage equal to that level 15 person.

This is especially a problem for low level tanks who will never be able to generate enough enmity to match the increased output of the dps that have access to all GCDs and oGCDs, while the tank doesn't even have a full enmity combo yet.