Even in road riding, for lightweight machines - such as the mentioned RZ Yamaha (~145kg) no rear brake is required,
& 'stoppies' are too easy.. ( & that RZ rear brake is literally 1/2 of the front set-up & too powerful anyhow,
- the same brake being shared by much weightier production Yamaha 1 litre+ 4Ts).
They do have uses, like not running backwards on hills, but are def' overkill otherwise..
It would be an interesting physics exercise to determine the optimum/ultimate configuration for the
quickest lapping G.P. bike power-to-weight-wise, but - "for sure" - it would not be a porky bloody 4T lump..