I know that departing from proven approaches for no sensible reason is
top
of the art but is there any kind of other rationale to make the thing
not
backward-compatible?
Of course, there's a reason: All minor modes since Emacs-23 (IIRC)
should turn themselves ON when called with a nil argument, so you don't
need turn-on-FOO-mode and you can just say:
(add-hook 'bar-mode-hook 'foo-mode)
The better part of this incompatible change is that it silently *fixed*
many people's .emacs since many people already used:
(add-hook 'bar-mode-hook 'foo-mode)
without realizing that this could actually turn the mode OFF in
some cases.
Stefan