For me the crucial difference between mini-buffer completion and pop is that one does not alter the layout of my windows while the other does.  Have a helpful list of suggestions to the right while my cursor remains undisturbed is very intuitive.  By contrast having a new window pop up moves _my_ focus but fails to move emacs' focus.  I would be very happy if, having configured emacs to pop a completion buffer emacs' focus moved there immediately.

/john