What you are labeling "bias" is simply a preference.
And we all have preferences based on our past experiences with products. And this applies to everything, not just cameras.
A "bias" implies an irrational preference, based on something emotional rather than logical. Some of us prefer driving Toyotas, others might prefer Fords. Some like Apple computers, some prefer PCs. Some people prefer a type of wine that others might not fancy.
When it comes to cameras, I doubt ANYONE selects a brand and spends lots of money on it out of some irrational prejudice. Normally, they have had a prior experience with that brand and liked it, or they just prefer the feature set, the quality or the prices. Or maybe they bought the brand because their brother owns it, and they can share lenses?
All of the above are RATIONAL reasons, so they become "preferences" and not "bias."
I think good photographers are the LEAST sentimental about their gear. You see them constantly switching brands when something better comes along. On each forum you will find some brand user bashing his own brand.
The "investment" factor comes later, after you have made the brand decision. For example, my first DSLR was an Olympus E510 with two kit lenses. Total cost $550. I intentionally spent very little because I wasn't sure if I liked the system. I wanted the "investment" to be something small that I could walk away from if it didn't suit me.
Howver, after two years of using that brand, I discovered it was perfect for me. There were so many things about it I liked that I then made the next step. I committed to the brand by buying an E30 and a few fairly expensive lenses. Now, I have $2500 "invested" and I'm financially committed. Now, it isn't so easy or painless to switch.
I think the "bias" part comes after the fact. After selecting a brand for good and loigical reasons, some folks feel the need to bash all other brands. And that is irrational. That is real bias.
The simple fact is every brand has strenghts and weaknesses, and one size doesn't fit all. since we are all different people. But every brand is capable of high quality results, so the brand bashing is completely silly.
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Marty
http://www.flickr.com/photos/marty4650/sets/72157606210120132/show/
http://www.fluidr.com/photos/marty4650/sets/72157606210120132
Olympus E-30
Zuiko 9-18mm
Zuiko 14-54mm II
Zuiko 40-150mm I
Zuiko 70-300mm
Zuiko 50mm f/2.0 macro