Why buy one bass that does everything decently when I can buy many basses that do one thing perfectly?
All of my basses play, sound, and feel different. A few of them are in different tunings. Two of them have flats, one has steels, and the others have nickel/steel blends. A Fender P-bass with flats and played with my fingers will sound radically different than a Stingray with steel strings played with a pick. I certainly wouldn't interchange my reggae bass with my Tool cover band bass.
I own multiple basses because I like to hold it over others' heads, and because I'm so ADD that I hate playing the same bass for too long. On top of that, I play exclusively 6+ string basses, but when it comes time for a session gig, you'd better believe that I'll be showing up with a Fender-esque 4 and 5-string. Every situation is different, every bass is different, every player is different.
If two works for you, then fine, stick with it. I'll never understand it, but I don't understand liquid soap either.