As far as building confidence goes, it is very dependent upon which area you wish for the confidence to be built. It could be said, that fllaunting ones fleshly beuaty in the manner of a pageant, which seeks to solely rank contestants based on sex appeal, builds confidence, in that they must be sexualized to be accepted by society, and that beauty should be idolized. This causes a strong misguiding as well, in the long run, brains gets the average person further than stunning beauty.
To me, it's just like the homeschool vs. public school argument of, "kids that are homsechooled don't get much socialization or experience of the real world." The argument almost works against itself. The whole purpose of homeschooling, is to educate. So if they recieve less socialization, and more education, then goal 1 has been met. In addition, it is a better experience of the real world in my opinion, as in public school there is competetion of fashion, in homeschooling and the real world, it doesn't matter. Confidence also stays stronger in homeschooled girls, than public schooled girls. I do feel that public school better prepares you for the sociality of the real world, to the effect that you have to deal with turds all day long.
It all depends on what area of confidence you want to build, as to which activity is beneficial.
Personally, I wouldn't allow a child to do a beauty pageant, as I agree with Imoss, it's a perverts dream.