We sort of addressed this recently so you may want to do a search. Several folks advised Brian Rupnow not to use one for milling while a few others said they were doing it and it was fine.
I think that if you only have a hammer, all the world looks like a nail.
In most cases a real mill vise has more capacity.
A mill vise can more quickly be adjusted.
A mill vise has detachable jaws allowing for a multitude of shop-made jaws which add all sorts of utility.
These same removable faces help you deal with the inevitable chewed up or marked up jaws.
Many mill vises are of the angle-lock design now days and as such will do wonders for preventing the work from wanting to pop up off the parallels.
These same knock off mill vises are not really all that much more expensive. When the right tool for the job costs about as one with which you can make do, why make do?