It's the lenses ...
That's a lot of cameras...
That depends. If your goal is to always have the latest technology then there is no reason for having more than one camera, but if over time you discover that you are producing something artistically interesting with a particular cameras, say the E-1 and the E-500, you won't want to get rid of those cameras even if their technology is "dated." "Dated" doesn't mean anything to these cameras. They will always (as long as they last) be able to produce the same artistically interesting shots that they could when they were new.
And I mentioned another "niche" reason for hanging onto E-420. It is very light to carry, especially with the 25mm pancake.
And while the E-1 is weatherproof, I now have the E-3 to take out in bad weather; so I have no plans to get rid of that either.
As to why I also have Pentax cameras, someone posted a note on Pentax explaining why he moved from Olympus to Pentax. He still liked the Olympus cameras and lenses, but it was depressing for him to remain in a system that was closing down -- the Olympus forums especially depressed him. Maybe I had something like that affecting me. Pentax isn't closing down. They tweaked the K-5 and came up with the K-5ii and K-5iis not so long ago which have their adoring fans raving. Also, Pentax fans have realistic hopes that Pentax is going to come out with something beyond the K-5ii & K-5iis fairly soon. Pentax people also like the K-50 & K-30 which have very late technology. I'm not interested in all that, but I was interested in eventually getting an E-5 when the price came down, but then I discovered that the lighter K5 seemed to surpass the E-5 in almost every respect. I suspect the E-5 has a better build & weather sealing but I don't know that. The K-5 is much lighter and a much better hiker's camera, IMO; so I may abandon my plan to eventually get an E-5.
I didn't move into all areas of the Pentax DSLR system as I did with Olympus, just their semi-pro DSLRs, the K-20d, K-7 & K-5. And despite those who denigrate the Pentax lenses, I'm getting pretty good results and have yet to acquire one of their best lenses. I can't say that my best Olympus lens produces better photos than my best Pentax lens -- but frankly I don't really notice that sort of thing. I'll take perhaps a hundred shots on a hike, go home, look at them in Lightroom 5, cull the bad ones and perhaps post 15 or 20 on my photo web site: www.lawrencehelm.smugmug.com -- sometimes fewer.
Life is good!