I find that I can go for quite some time with just a mp and a 50, either my zm sonnar or my summilux asph. I see times where I'd like a 35 or a tighter 75 fov, but I find ways to compensate when I compose. I recall reading the paradox that limiting oneself artistically can function so as to open up limitless possibilities with a specific focal length, a paint, a poetic form or whatever. I *think* it was Ralph Gibson who said that, but I am not sure; whoever made the claim though was correct in my estimation. As practice makes perfect, according to the old adage, so it would suggest logically that if you work consistently with a certain focal length, you'll get more comfortable and proficient with it.
This being granted, I do think there are really good artistic/functional reasons to use other focal lengths. I am glad I believe this, too, as I'd be disheartened at times if I couldn't use a 35/75 combo instead of my 50. Ah, the focal length sacrifices some make for Art!