I have what I have because I enjoy using them. Stuff that doesn't get used gets sold or donated. Well, eventually...
When I first got started in photography I had an F1X bag stuffed full of crap I wasn't using. It took me a couple of years to figure out I wasn't doing my back or shoulders any favors, and promptly sold the lot. I settled on one camera and one lens, and that satisfied me for ten years, until the camera died. (Today I only carry one or two bodies with lenses mounted, and I don't spend a lot of time trying to decide what to carry that day.)
Then I discovered cameras on ebay. An awful lot of those bodies were bought simply to satisfy my curiosity about them. Most are gone now, though I still have 30 or so bodies I use in some sort of haphazard rotation.
I have a Canon FD system for sentimental reasons, and is likely my most complete kit.
The EOS system is for when I want autofocus. The EOS M for when I want digital.
The N80 and one zoom is a great little camera, and is probably a candidate for the donation bin.
The Contax system scratches an itch I've had for 20+ years.
I have a few medium format bodies that, while I haven't used them in a while, I still value and intend to use.
I have a large assortment of Minolta gear that will probably get the axe in the new year, except for a pair of XD11.
If I had to keep only one system, it'd be a very tough choice.