The percieved advantage of new = better.
To my mind new = untried / unknown.
I've stuck with the same manufacturer for just about my whole time racing RC (1st Kyosho in 1987!) and have only moved around when they decided to concentrate their efforts away from 1/10th off road. For me it is knowing how the cars are likely to behave and that they will be quality items.
In the "dark days" I pretty much ran just 1 4wd (Predator) but changed the 2wd a few times as nothing really suited me until I found the B3 but I alwasys changed in the off season so I had chance to get to know the car before selling the other one.
I guess that puts me at the extreme other end of the spectrum to those you mention but I can name quite a few guys who have run 10+ years with the same manufactures in my region so it's not that uncommon.
I guess the simple answer is buy Kyosho and don't worry about the rest
