Getting your 'first of a species' done is probably the best reason to get a head mounted. You only shoot 'your first' once and probably has a wonderful memory to go with it.
Getting your biggest head done comes a close second, but I have set up 'biggest heads' before now only for the owner to go out and shoot a bigger one the very next week!
Malforms are another great attraction, as every one is different.
Lakey