Strange logic, I personally would not become a super-relief is I have the choice between regular hirer or regular relief driver; will earn less money unless of course I 60+ or 70+ then difference case.
In any case, it is nice to know there are such people around who do it on a frequent basis.
I suppose the company super-relief also only take full-day jobs only. Last time, my ex-hirer went on leave, I (day-relief) also had to go on leave too so that a super-relief could takeover the taxi during his absence. And the super-relief park the taxi at his place until time to hand over.