I really don't think the reduced TEM is much of a factor as a partially destroyed/damaged building may be even more defensible than than the if in pristine condition (e.g brick is brick, wood is wood). However, the ability for it to be reduced to Rubble is probably enhanced. In a playing of RB we gave gutted buildings a -1 DRM for rubble creation purposes only, in essence the reverse of a fortified building modifier. It was implemented with little difficulty and attained the desired result without drastically changing the game play or do I expect, the outcome of the game.
As for progressive damage inflicted upon a building, I know most computer games use this approach, but I believe it would be rather difficult to implement in a similar fashion for ASL. Now I could see a "Damaged" counter being placed upon a building that let's say received a "K/#" DR result during the normal rubble creation process that would increase its chances of being reduced to rubble, but do not believe it would reduce the inherent protection afforded by its building materials. The possibility of a "normal" building being gutted (ala. FB) that have been on fire during a CG should be implemented with future CG (including the remake/reissue) of RB as I believe this gives a reasonable amount of randomness given to building destruction. Again, if the increased possibility of rubble creation is applied to gutted buildings & perhaps a form of a "damaged" annotation, I believe it would reasonably reflect progressive damage to structures and at the same time keep the rules at a minimum and ease implementation.