From what I've heard from auto engineers I know, using the battery as part of the structure is not really done. Transfering mechanical stresses to the battery is something you just do not do.

Additionally the battery must be protected in the event of the crash, so its casing must survive intact.

I mean, it's possible that some manufacturers might do it a little bit to put it on the marketing brochure, but the additional design headaches and safety concerns mean that there's just not that much to gain.