OK, I might be getting my types muddled up here... been flying a desk too long now..
IIRC though, comparing TO pitch mode and Level Change, do the mid vintage 737s not behave identically in pitch as far as acceleration schedule etc, but there is one difference:
TO will not auto change from IAS to Mach number.
So, if you had a continuous climb and a combination of circumstances that precluded VNAV, then with sufficient lack of attention you could exceed MMO.
In my last outfit, IIRC, there was a positive check that we were no longer in TO mode on passing FL100.
I might be getting mixed up with the prophylactic jet though.