You have a good point. 
Also, I realize that, if players wish to use it as part of the profile (as it is evident that some do), they will eventually do scripting/custom client module workarounds for it anyway.
The only way to prevent it would be to make some strict server-side rules like “avatar can only be changed once per day” or something. And I don’t think that would be a good idea at all.
There is no extra stress/difficulty server-side. A bit of refactoring.
One question:
For mail (and other similar things to come), where the avatar is shown, should it display the avatar used when sending the mail, or should it always show the present avatar?
I think I am leaning towards showing the avatar used when the mail was sent, and not updating it even if the character changes profile.