I think, like darned near everything else, it depends.
I have a very modest collection of sample libraries, and yes, if I were earning more with them I'd get more of them. But I try to make sure income is greater than expenses<G>!
Some of the purchases were made out of curiosity, I have Albion One, OE 1&2, Swing and Swing More, and Metropolis Ark 1. It has been a bit of a struggle learning to use these libraries, and common sense would suggest that I should have given up after the first one<G>. I'm glad I didn't! It isn't that one of them suddenly made sense to me (oh that it were that easy), but rather my continued experiments with all of them finally paid off. And you know what? They really are remarkably different, in spite the common thread.
Same goes for straight multi-sampled libraries, for example I use VSL SE, Chris Hein and Cinesamples for orchestral brass and winds - they are very different sounds, and very different workflows. To me they compliment each other, and I am able to do far more with the combination than I could with only one of them. Same approach for strings, I use 8Dio Adagio/Aggitato, Cinematic Studio Strings, and VSL SE.
Sometimes it is as simple as trying to accomplish something with one, getting no where, trying a different library and it works - probably not entirely because of the library, I think sometimes it is just the change.
I would guess that folks with more extensive collections find the same things to be true, just on a bigger scale. I'm sure there are collectors out there too. The more the merrier, just as with guitars the collectors make some of these things more affordable for the rest of us.
And for the record, I do own several guitars, and I think they are all different, no, I know they are. Some are work horses that get used on most of my projects. Some are specialty items that get used a couple times a year (maybe even less in the case of my Mosrite Ventures - but when I need that sound the Mosrite is the only guitar that gets it.)
Now I did learn a valuable lesson about buying on credit. Many of the earlier guitars were purchased on terms, and it caught up with me. Fortunately I survived, and learned my lesson. Nothing really wrong with that either - some of us just have to learn things the hard way<G>!