Without a doubt, the bodies on RTRs are the single most costly item. But, and I may well be wrong, I would doubt if a plain white version of the body could be worth more than 20-25% of the complete car. Without paint, tampo work, interiors (which could also be offered separately!) and fancy packaging, bare white shells in a simple blister pack wouldn't carry the same cost as the body on a complete RTR.
Would enough sell to make it worthwhile for the manufacturers? I don't know. They don't need to invest in anything, just run additional copies of bare shells (and possibly bare interiors too) which actually lowers their price-per-unit cost. Then again, they have to package the bare shells and also fit them into their distribution scheme.
Will it hurt sales of RTRs? I don't think it would. Very few people buy the RTR just to get the body for scratchbuilding purposes. And I doubt that a significant number buy a duplicate RTR just because the body on the first got damaged.
For me personally, I would still buy the same number of RTRs because I want the factory liveried versions and intend to run the cars anyway. But I would buy additional bare shells for scratchbuilding, and would jump at them because it isn't easy to find good 1/32 plastic models and plastic kits aren't cheap anyway. But I'm probably not the typical buyer.
I imagine the manufacturers aren't doing it because they just don't perceive that the market for bare shells is large enough.
