I've actually done it a few times.
I've had Sony, Canon and Nikon.
My Canon 400D was my last major change. I had a nifty fifty, a Sigma 18-50mm (basic) lens, a Sigma 18-200mm OS, Speedlite 430EX, Sigma 70-300 APO.
No matter what I done, it always took CRAP pictures!!!!
Went for the Sony a300, which always takes superb pictures.
I like selling my gear and buying into different systems.
I believe it's all part of the fun.
I've also had a Nikon D40. Which took FAB shots.
I sold the D40 due to there not being many lenses available for it. Although it's getting better....
Part of a hobby for me is buying new gear, and I can't always afford to buy top of the range or even new stuff, so although when I do sell up, I usually lose a little bit of dosh, but I very rarely buy my stuff new so I don't really suffer huge loses.
Here's a good example of what I'm trying to say:
I'd like a 70-200mm F2.8 lens, which the Sony version is around £1200
I am NEVER going to have this much money to spend, but I may one day after some saving and good luck etc (!!!!) have say £500 to spend.
If I sold my Sony gear, this would give me the opportunity to get for example
say a Nikon D80, nice 'kit' lens and a Sigma 70-200mm F2.8.
This does open up a few more concerns though, so chances are I won't ever do it!!!
Because of my pretty tight budget, I have to sell stuff, in order to get new stuff. Now if I were made of money.......