Designer is important, the roles of Doctor and TD are still being developed somewhat and you're right to notice that they don't seem to have much impact at the moment.
Maybe you could relate the doctor to the regeneration of the health of your drivers. But the formula should start with a minimal value of 4% or sth, so that you really have a chance to train your drivers from the beginnin and not buy for every season a new one. Maybe the regeneration rate could be increased to 6% or 8% for a maxed out doctor. not sure about that max part.