I think in a lot of cases, it's fairly simple: with a "softer" lens, you can get the smoothing with minimal effort, and you virtually eliminate the possibility of producing skin that looks unnatural and "plasticky". With post-processing, there's frequently a pretty narrow range between annoyance from blemishes, and annoyance from plastic-looking skin. In some cases (especially with older subjects, for one example) that range narrows to none at all -- the skin looks completely fake, and yet the blemishes still look pretty excessive.
A "soft" lens can often do what seems impossible in post-processing, and with the same subject under the same lighting, the blemishes become much less visible while the skin still looks entirely natural. For example, here's a pair of pictures taken at f/3.5 and f/2, but with the same lens and nearly identical lighting (taken about 30 seconds apart or so).
At f/3.5, there's clearly more DoF, but the skin just below the eye is in pretty decent focus in both cases -- but in the f/2.0 shot, while the skin still has enough texture visible to look natural, it still has noticeably less than at f/3.5. Of course, given that this is a 9 year-old, you don't expect to see a lot of deep wrinkles in either case.
Especially given that we're looking at 100% crops where, I doubt that most people would think of the f/2.0 shot as being terribly soft -- but I think you can see where a lot of women in, say, their mid-30's would prefer the look of the first to the second.
I should add that in a lot of cases, you can accomplish something similar even with a lens that doesn't get particularly "soft" at a wide aperture. The trick is to find areas that are relatively wrinkle- and blemish-free that will be in sharp focus, and let the minimal DoF take care of "blurring" the rest. Since this is something that's going to happen anyway, it usually looks quite (entirely?) natural -- the real trick is getting what you want to in focus without getting the person into a strange/awkward-looking pose.
In case anybody cares: Sony Alpha 700, Minolta 85/1.4 G(D) lens, f/2.0@1/60th, f/3.5@1/30th.