If there weren't ridgeless Romulans, then Spock wouldn't have been able to hide within Romulan society.
And who is to say that Star Trek aliens don't have regional/racial differences like humans do. We could go as far as to say that they even have racism like humans do, and that there is social stratification based upon race. Maybe there were ridged Romulans in the upper echelons the whole time, and we never saw them.
Also, to me, the Abrams Romulans still looked like they had ridges, Abrams had just toned the look down quite a bit and brought them to something midway between TOS and TNG (like he seems to have done with Klingons). They were still recognizably romulan to me. I can't wait to see what he does with other Trek races (especially Andorians