??@#&??
Where is that rule written?
You buy and use whatever lens works for you and gives you the type of photograph you see in your mind's eye.
I have several Leica lenses I enjoy using but right now my favorite lenses are the Nokton 50/1.1, the ZM Sonnar 50/1.5 and my ZM Planar 50/2. Every single one of them shows the same view a little differently, but with very high quality.
Of course next month I may be enchanted with my Leica Summilux 50/1.4 and my Leica Elmar 90/4. All depends on what I am trying to photograph and what film I am using.
But I have never, never bought a lens just because of the name on the front ring.