As opposed to vehicles exploding instantly upon being damaged enough or striking something, some of us were talking about having crumpled/dented models for severe damage. So if you head-on hit a mountain, your car turns into a crumpled mess, but doesn't explode... until the fire hits the fuel tank ;)

This would require a complete re-write of model rendering code, as well as a re-do of every existing model to support such a feature. This is incredibly difficult to do in MC, and has significant performance issues due to needing multiple model states. While the visuals would be nice, the execution and performance hits would be so significant they would easily outweigh them. No support.