I really can't see any drawback to the make before break switch other
than cost and availablity. As long as you have appropiate fuses or
circut breakers in the system fire should not be an issue and 99.99% of
the time you will only be switching from a low battery to a good
battery. and the two batterys are only connected for a few milliseconds
which may not be such a bad thing as it will slightly slow the voltage
transition to your instruments.

Brian