From what I understand about how they're able to do this they have to limit it. Because for a skin to be used it has to be hashed (Approved) by Mojang's servers and so what they do is have an account that acts as a bit of storage account in away where all the skins go through. The problem is Mojang basically only allows you to change your skins a few hundred times per hour before they just stop you. Meaning the whole function would break under heavy load if a lot of players just switched back and forth and once it breaks everyone would be stuck for awhile. My understanding isn't perfect but that's somewhat the gist of why at least all players, for now, wouldn't be able to do it.