[Left]: This woman is not actually a doctor.
There is a certain art to female television commercial casting. What it basically boils down to: the woman has to be attractive, but not TOO attractive.
For example, if the girl is ridiculously hot, it means the commercial is a farce. There are unquestionably hot girls in the Axe, Tag and Right Guard commercials, and all of them are farces. The exceptions to this are commercials featuring makeup or beauty products, but this is normally by accident, or feature “respected” hotties. Beyonce (good) vs. Carmen Elektra (bad).
The more common woman featured in commercials in the subtly hot girl, especially in birth control commercials. I'm not sure why this is; maybe BC companies figure people will have more sex if a good-looking woman is involved. Prominent examples are Yaz (the “I'm a doctor!” woman is foxy as heck) and Minerva (even though the redhead has like 14 kids in the commercial). This does not apply to condom commercials, which normally take from the “very hot” category (Trojan).
The “subtle hot” model is also sometimes cast by accident. For example, for years Dove had a campaign where they cast “normal looking” women in their deodorant and beauty commercials. This would have been a good idea, except that almost everyone they chose was still hot. Some might have been thicker than a normal television model, but pretty much all of them had great complexions and, to use a medical term, nice cans.
And finally, there is the “we have a message” commercials, which just have normal people because the story is more important. Gastric bypass surgery commercials have this sort of model, as do weight loss commercials, oddly. You would think that Jenny Craig commercials would have nothing but 10s, but more often, you have frumpier people involved. Same with Nutrisystem and the male workout videos, although they often rely on former professional athletes. Charitable causes – donating to the blood bank, fighting against juvenile diabetes – are also normally exempt from hotness concerns because of the message involved.
With the last category, I get the feeling that actual models aren't used, which is probably part of the reason why all different looks are incorporated. The other reason good-looking women are almost always cast is a practical one in my opinion – There is no economic incentive to cast a less attractive one.
If you go into a modeling agency, obviously Nikki Taylor or Carmen Elektra will set you back more cash, but there probably isn't much monetary difference between Unknown A and Unknown B. I tried to argue this point in Women's Studies classes once, but it got kind of brushed aside without an adequate answer. If you go to a restaurant and prime rib is the same price as sirloin, then there is no real incentive to get the sirloin.
To end this randomly, this whole subject percolated in my mind about six months ago, as I seriously thought about launching a web site called “That Chick In That Commercial” with a buddy. We didn't, mostly because I thought there had to be some site out there already like this; a la Knocked Up, when Seth Rogan finds out about Dr. Skin. But there are dozens of commercials out there with fetching women that nobody has heard of, but they'd like to know more about. (My current favorite would be the