NESparky,
In my experience, the fasteners fail before the wood. I'll find 60 year old services on "one bys" that are held in position by the mounting screws of the panel, with, maybe, one or two of the original masonry nails left in service. The masonry gives way. The wood and the box are OK. Panel and sub panels are held in position by the wood, not stressing the panel-conduit-panel bonds as badly as if they were hanging only by their raceways and cables.
Another value of the backboard is as a thermal break to help minimize condensation; and as a moisture barrier to prevent corrosion. Most older dwellings around here have basements, and the below grade masonry will dampen when covered even when the basement is experienced as "dry".
Lastly, if there is enough moisture to take out the wood, it'll take out the panel anyway.
Al
