I prefer simplicity, so for me, there is real appeal in having just one bike. The way I ride would make that easy too: I'm a year-round commuter and like to make longish, rambling weekend rides. My Bianchi Volpe comes pretty close to being "the one" for me.
I do have a second bike, though, and it gets ridden pretty regularly (an IGH Omafiets with all the trimmings). I think I'll always have two bikes so that I have something to ride if one of them is unavailable for some reason. In spite of my stated preference for simplicity, I might even end up with three. I'm considering adding a folder to the fleet.