DevonDiver
N/A
The downside of sidemount/IDs of course being you can't access the gas from a tank with a bad 1st/2nd stage.
You can, but it is admittedly not so convenient.
Most sidemount courses teach feather breathing to access gas if you've had a free-flow. It's an easy technique to master.
Higher level sidemount courses also teach the procedure for swapping regs between cylinders. This'd only really be necessary in very rare circumstances. It's quite easy to achieve, given the location of the valves.
It's erroneous to assume that you 'write off gas" because of a regulator failure on sidemount.