I know at least two reasons:
1) angelozip wants to check if 34xx firmware has better GPS accuracy with 37xx bootloader.
2) I consider flashing 37xx with 34xx boot.bin quite dangerous. As far as I remember 37xx prints "System Software Missing" in white color while being flashed with 34xx RGN. I have only one explanation for this fact: 34xx boot.bin crashes after flashing 34xx bootloader and the device reboots immediately and executes the new bootloader, which in turn reads the old firmware and finds that its HWID is wrong, then prints the message and enters pre-boot mode. Then updater.exe continues flashing the rest of RGN with help of bootloader (yes, with some nuvis you can remove boot.bin from RGN file and still flash it normally, as bootloaders can flash some regions).
Now guess what happens if for some reason 34xx boot.bin crashes a few moments earlier, while flashing 34xx bootloader?
Also once I've bricked 34xx by flashing it with 37xx boot.bin.
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