OK firstly, I did lose 850MHz when flashing the standard image on, so you definitely can lose (but never gain)
The Radio PHY in the phone will be split into two parts, the 2100MHz part and the 850MHz part (or a 900MHz part).
So on the normal wildfire radio image, there's a driver which is likely compiled to drive a 2100/900 radio, and when we run that image on the 2100/850 phone, 2100 remains working because this part of the chip is identical. The 850MHz part dies completely because this driver is not expecting it.
I know how lazy microcode developers are - it's very rare for anyone to go to the effort to make microcode images to support two subtley different chips, instead they will compile two versions of the microcode with subtle changes for each one. Only one bit in a register need be set wrong and the whole lot comes crashing down!
Now there could be a scenario where the radio is a 100% hardware solution, and there is no microcode for the UMTS radio, in which case there's a better chance that having the wrong radio image is not an issue, as the chips themselves are more autonomous. But then - What does the radio image do? In my experience of working with 802.11 Radio PHYs: Not likely. These chips are almost always highly software driven and must be running the correct code to fully function.