I justify my collection like this.
A. They're all paid for from gig money- it's either pay the taxman for the income or spend the money on legitimate equipment for the business
B. They do different things, and I could even justify having more multiples. For example, I only have one Precision bass and it's got flats on it. Having another with roundwound strings would be a good move and entirely justified for the totally different applicatons
C. I have two basses essentially the same (a pair of Lakland 4-94s) as that's my primary gig bass. Having a spare bass on stage is just smart insurance. Having one that matches the main bass as much as possible means that there won't be big differerences in gain levels nor in EQ settings if I switch. That makes my life easier and the sound guy doesn't have to tear his hair out if I were to switch to something totally different.
D. At the same time totally different can be exactly what I want. If I swap the Lakland for the fretless it's because I think the music is served by that change.
So I have-
I. A Precision with flats, it's perfect for a lot of gigs, and not really right for a lot of others.
II. Two Lakland 4-94s. Versatile and wide range of useable sounds available. My primary stage bass for the last ten years, so I have a backup of it.
III. A fretless- duh, it's fretless!
IV. A 5-string- duh, it's a fiver!
V. A Sting Precision Bass- OK, it's just a great sounding bass that feels perfect and looks pretty cool. With a set of bright roundwounds it's my ultimate rock 'n' roll bass!
VI. A Guild Pilot- OK, this was supposed to be by roundwound P bass, and I always liked them when I was selling them new. This is an indulgence, but as my buddy with 35+ guitars (those skinny stringed things...) says "Hey, I don't play golf!".
That last comment is another critical factor. I don't spend money on cars (beyond maintenance and basic transportation), motorcycles, ball games, titty bars, booze, cable TV, golf, movies, model airplanes, Lionel trains, guns, fishing, boats, travel, etc. Other than two really good bicycles (one is sixteen years old so it's cost me $75/year, or about five cents a mile), I don't spend much money on hobbies. And I've made a lot of money gigging, so it's easy to justify a stable of basses and amps.
jte