Ergonomics and signature are important. I have two 50s and two 90s and two 25s. Ultimately I will have one more 50, at least, and another 35, probably a 2.8 Summaron. Maybe I'll want a faster 35 than my Summicron too. The quick light lenses like my 50 Summicron and CV 25 are for fast shooting and carrying. My Zeiss 25 and 21 are for architecture and shots where sharpness might matter a lot. I couldn't live without the Zeiss C Sonnar. My collapsible 90 Elmar has a little haze and gives lovely dreamy shots. It's heavy, but compact. The 90 M is perfect but a little bulkier. I almost never leave the house carrying more than three lenses in my bag, and most times I go out with only one lens.
Ergonomics goes for bodies as well. The M5 is bigger and heavier but great for the 21 and 25 as it is easier to keep dead level, hand held, and easier to keep the film plane parallel to the face of a building. My father would come to my house to repair something with just five tools in a white bucket. He never needed what he hadn't brought, and that was a lot, all neatly arranged in his shed. Photography's the same.
