Hi Doug,
In one sense you are correct, most planes do not need differential
Many of the superior pattern designs do well without it.
However, it is a competition craft where differential can pay off
best. it can garner points in the roll maneuvers by making axial
rolls true without input from the pilot.
Mystery is why one design does well without it and another good
design may require it desperately.
What would be interesting would be to conduct an investigation
to determine the cause for the need with one and not the other.
But a wonder is if the effort is worth while when the use solves
the problem neatly and requires little to try.
Good luck and be good,

Hal deBolt