According to Bengt Björkbom it was Christer Strömholm who once said "one camera, one lens, one film, one developer" and Bengt has written some nice stuff on that.
I really like the way that constraints can muster novel thinking. I have this long-term project: 36 photos. I go to a street with one roll of film and walk the length, my challenge is to make 36 pictures before leaving (it helps if there's a cafe!). Sometimes the challenge is to find 36 photos, sometimes to take only 36. It's always fun by the time I'm done.
One camera, one lens has great emancipatory potential from the burden of GAS. Those of us on a shoe-string will recognise this. But then most of my favourite photos were taken when I only owned one camera / lens. I still have more fun by leaving most stuff at home.