A pen-knife in use wouldn't be restricted to trimming quills. Like a Swiss Army knife, a small knife (originally carried in a sheath rather than folded up like a SWK) would be handy for all sorts of things.

I'm guessing, but pen-knives may have been larger in the distant past than they are today. A modern-day butter knife, for example,is pretty "wee" compared to a hunting or butcher knife.