You're correct, the mill does have a quill lock. The only issue with the spring retention type as you listed is that you've got to make sure that lock is 100% effective. If it isn't, you're going to force the quill downwards when you want to do a tool change.
You might want to look at a means to lock the quill other than by the existing method. I decided to go with a small butterfly air impact wrench design for my draw bar as even though I do use the Tormach quick change system most of the time, I do swap other holders when I need to.