I think touring car bodies are pretty big compared to the cars - and it's also a tuning aid. Neither of these things apply to off road and it's not possible to use any other shell than one designed for the car (generally speaking) so most people buying the 511 will be buying the 511 body set at extra cost - along with wheels which again aren't really a tuning option, unless you want a different colour.
Not a 'dig' - I like Tamiya, and I'm sure the car's great - but the 'big idea' behind the way the 511 is being marketed stumps me, it really does - the car has no identity.