Necessary? No. I've owned a ~500 pound Ibanez SR406 string that was perfectly adequate for any job. Fairly quiet pickups, good output, decent playability, all fine. The sound was workable, nothing to write home about. Would it have taken the abuse of 10+ years of heavy, continuous use that my 1999 Warwick Thumb 6 string has taken? Probably not - the Warwick's hardware is obviously more expensive and you can tell it's built like a tank. The Warwick's sound is also much better, in that it cuts through the mix whatever I play.
Given all that, I'm in the process of getting an entirely custom instrument built for about the same cost of a Warwick custom shop build. Why? I have a specific image in my mind of what my dream bass would be, and if someone came up with a production model of it (Ibanez got fairly close with the new Adam Nitti bass, I considered ordering one of those) I would've jumped on it.
But more specifically, I've been playing bass for almost 20 years now, at a professional level for about 10 (meaning in my case that I make some money off it, but not my living - in any case, I am very active as a bassist.) I feel I owe it to myself to get my specific, exact bass and I've got the financial means to do it now. I'm getting it because I want to, the luthier in question will carry out my exact requests in terms of woods, finish, electronics and so forth, and I know I will be getting a world-class instrument which will be super-versatile.
Do I NEED it? Categorically, no. I've got my Warwick for literally any musical situation - I guess I could play in my punk/thrash band with it even, but it would look a bit silly so I have my Stingray 4 for that. I guess it's nice to have a couple of basses in different tunings, maybe. But unlike some other guys&gals here I haven't built up a massive arsenal of basses - I only have 2 I use with any great regularity, another cheap Peavey I customised as a project which sees a decent amount of use and my first-ever bass which I couldn't sell for sentimental reasons (also because it's beat up as hell and completely covered in stickers and graffiti).
However, I want it, I can afford it, and I'm willing to pay for all the details I want at a custom hand-made level from a reputable builder.
tl;dr not necessary but why not.