I have used ABRSM Jazz exams for several reasons
1. If a student shows a particular interest in Jazz/improvisation
2. If a student is slightly losing motivation with the classical piano exams
3. As a sideline skill to those who prefer a holistic experience in their lessons rather than racing through exams
Whatever their level of playing, I generally start them off with grade 1 Jazz so they can understand the principle of the exams, but I don’t always do all the grades after that.
Or I have started students with grade 1 Jazz and they have never taken classical exams.
Some students find Improvisation easy and exciting so don’t have a problem starting their exams with grade 1 Jazz Piano. Others definitely need to be above grade 3 piano before they would cope with Jazz exams
it really does depend on the individual