JeRuFo, do you mean good reasons not to switch your primes to zooms? Because I am struggling to find good reasons to switch. There's no major advantage in terms of size/weight, it can save a little money, but if you already have the primes, that's out the window. It is more a convenience of shooting, not of ergonomics. If you want to zoom in or out on your scene, and you are going to be restricted (meaning your zoom legs won't be able to move you), then the functionality of just leaving the zoom on the camera might have some positive allure for some.
But as good as the zooms are - and they are very good - the prime lenses for the XF system are superior optically. Perhaps just a tiny bit in some cases, but in every instance I've tested and compared them, the prime has always won. They are also faster, which may be helpful, and again, the size of a 35mm/55mm/80mm is not really all that different from a single zoom lens, since the zoom lenses are so large.
I'm a bit biased toward primes personally, but aside from the above, it really is a subjective choice in the hands of the user.
Steve Hendrix/CI