True, but if you are building a McLaren replica it does not leave much room for manovre even if you build a spaceframe the outer sills are so thin you need the structural bolsters down each side of the seat even if they are tubular and boxed in. You would also me mad not to take advantage of being able to put a strengthening structure behind the drivers seat.
I aggree there are other ways to do it but it is making it more difficult and less structurally ridgid. I know I have done a load of FEA analysis on various options and believe me they used the best and lightest method.