The only time I can remember never going ashore was many years ago when Princess tried summer cruises to the Mexican Riviera. I think it was in July or August and I remember walking down the gangway and turning around and walking right back up--the heat and humidity was THAT oppressive. Of course nobody else went ashore either so it was just like a sea day.
But when going to Mexico, which I do a lot because it is so convenient for me, many times I will skip one or even two of the ports for the very reasons you mention. But I've found that there is a point where "quiet" can actually turn into "lonely" :-)