I know there is no one answer, but I am thinking that owning fewer basses means that I can learn about out of each one instead of owning a bunch of basses I rarely touch. Frankly, most folks who have multiples, have one or two favorites that get most of the playing time.
As I said there is no one answer. Some people love to collect. Some may have a few basses for very specialized uses. As for me, I only have one gig, and I suspect anything that stays in tune will work for me.
I'm sure it's been said, but...
Depends on the goal. If you need to push your chops to the absolute eXtr3m3 limit, then I suppose it would be useful to focus on a single bass. That ain't me! All I play is 5 string with wide spacing... but outside of that, I have basses with neck profiles that range from "slim'n'fast" to baseball bat chunky. I like to stay fluent on all of 'em... first because why not? Second because every so often I do a sit in and use someone else's bass. And yeah, I can still find my way around four string OK when I have no choice, even though I haven't owned one in a decade.
rigid and limited? focused? practical? 
