There are many, many portraits out there shot "head on" - fashion and magazine photos often use this kind of pose. Frankly, their models can pull it off.
For the majority of people out there, a straight on pose will most often add a certain bit of weight to the face, whereas an angle to the face can reduce "flabbiness". Most people paying for a photo want to reduce their weight on camera.
Additionally, lenses that have significant barrel distortion will have an extra effect on the "flabbiness" when used head on. This effect is more minimal at an angle.
Some people can pull it off, but its really about the lighting. Its a decision you have to make as a photog - what looks best for that person.
EXCELLENT references on the subject really include the recent questions and answers about broad, short, split, butterfly, and Rembrandt lighting (mostly by Jay Lance Photography) -which cover not just the lighting but the facial pose as well.