I think any use that the user deems necessary is "legitimate", as long as he doesn't have unrealistic expectations of the knife, or of the maker to stand behind it.
A knife is a very simple tool, that can be adapted to a very broad range of uses. All use causes wear to the knife; some uses cause wear at greater rates than others.
The sorts of knives we typically discuss on this site are generally field tools - they are meant to be carried around on one's person. This would be as opposed to bench tools or components in a large kit of tools carried to a jobsite, etc. So an EDC/field knife is an inherently *general* tool. The whole point of such a tool is for it to be adapted to the task at hand.
I am not arguing that all knives need to be able to withstand abuse, or even that the ability to withstand abuse makes one knife better than another. What I am saying is that there are situations where abuse becomes simply use; and in those situations a knifes resistive capacity is an important attribute. It is just another layer of utility.