The answers seem to lie in a book by author, Paul Bloom, entitled:
How Pleasure Works. The New Science of Why We Like What We Like.
I am a truly addicted reader and, contrary to legend, I do read as much non-fiction as fiction. I am currently making my way through Paul Bloom's book and it's giving me fresh insight into the endless brand wars between otherwise rational photographers.
Leaving aside our initial buying decisions for a moment, the book makes the argument that once you've chosen something and received it the object attains an "endowment value." Basically it means that the object is no longer an anonymous and replaceable thing but it is now yours and by the very nature of you possessing it the object has more value to you.
Bloom references a study of market valuation which was done to bolster this idea. Essentially, a person is offered an object of value for a set price. His example was a coffee cup for five dollars. Once the person had committed and bought the coffee cup she was then offered six dollars to sell back the cup. In general the persons in the test refused and considered the cup to have a much greater value now. While the transaction would have netted the test subject a quick dollar of profit in mere seconds they were emotionally unable to logically understand the objective value proposition. It seems that the endowment valuation is at work in every purchase that we, as consumers, commit to.
A second issue is the power of having made a choice. It seems that making a choice between random but identical objects invests the chosen object with more value and degrades the value of the identical item not chosen. The test described in this example was a bar game in which you have three identical coasters. You hide one coaster and then ask the test subjects to make a choice between two remaining coasters. Remember that the coasters are identical. The test subject is still asked to make a choice.
After choosing the tester brings out the third identical coaster and asks the test subject to choose between the previously declined coaster and the newly revealed coaster. Almost without exception the subjects chose the newly revealed coaster signaling that the previously rejected coaster was less desirable and so not a good choice.
It seems that once humans make a commitment to own something both the power of choice and the (irrational) endowment of value come into play and cause us to defend our choice and denigrate the unchosen objects. Other studies bolster these findings and speak to their near universality, not only amount humans but also among some other primates.
There is another related force at work which keeps us "loyal." We, like almost all species, are more comfortable with a known thing or experience than a new or unknown thing or experience. In happy relationships satisfaction with partners is shown to increase over the long term specifically because our partner is known and safe. Safety is the basic parameter we are all trying to achieve so we can continue living and safely pass on our DNA. As a result of millions and millions of years of evolution the compulsion to choose safety over implied, additional, but unknown benefits is an overwhelming one. The longer we work with a brand the more comfortable we become with its operation and even its quirks, even if they are demonstrably inferior to the products of competitors. That familiarity and understanding of "safety" tends to cement our relationships and, by extension, our brand loyalty.
In short, you like your camera better than my camera because you chose your camera (for whatever external or rationalized reasoning=marketing? Group persuasion? ) and you like my camera less than your camera (even if it's performance is identical) because you initially did not choose it. You have further cemented your positive appraisal of your camera through familiarity and your dismissal of my cameras choice because it is relatively unknown to you and therefore relatively unsafe.
And this is why Canon lovers love Canon cameras and Nikon lovers love Nikon cameras and etc., etc.
I know. You are an engineer, I.T. guy, math guy, and you think you made only logical choices and are immune to the psychology of choice. Paul Bloom and I think you are wrong.
I haven't gotten to the part of the book where relative rewards of new risk taking are covered but I can impute that it is risk taking that moves the species forward in an evolutionary way by uncovering the risk/reward math involved in having new methods or efficiencies made available. Either that or I am also making the same kinds of rationalization as above to bolster my camera choices.
Multi-system owners? Either they want to have the right tool at hand for a specific job or, more credibly they are trying to cover all of their bases so they can enhanced their perceived safety and ranking in their tribe.
The book and the research are eye-opening and a bit humbling. But it all boils down to sex and survival. You might find it all interesting.
http://www.amazon.com/Kirk-Tuck/e/B002ECIS24/ref=ntt_athr_dp_pel_1