I think one lens is a great exercise. It forces you to see differently.

If you have a comfort level with a certain lens you should make a habit of taking only something else.

If you like wides, then take only a telephoto.

It can force a scenic photographer to start to see tighter closeup and visa versa.

When I did darkroom work and had say, a 20x24 set up, I would make a test prints with just 8x10 to get the exposure and would have just a tight shot of the face. And I thought, hmmm that 's pretty cool.

It taught me to start to shoot tightly on the face occasionally, which is something I never did before.

It can elevate you work a lot because your first thought is often to take a rather mundane shot of things, and close cropping can open a whole new world of interesting work.