I remember Kevin Smith talking about his first stab at dieting, following the instructions of the "guru" that got Penn Jillette lean: basically, he ate nothing but potatoes for something like 3 months.

The point wasn't that potatoes were particularly good for you, but to "reset" one's attitude towards food, eating only when hungry rather than for pleasure or to deal with stress - because after a few weeks, one is so tired of potatoes that they'll forego unnecessary eating.

In this sense, the potato diet is probably useless in itself, but might work in that "reset" role.