I think their point is to be as obnoxious and attention-getting as possible. The whole thing is very tongue in cheek in the sense that japan is a very reserved place and to be outlandish and obnoxious pushes you to the fringes of society very quickly. I don't think they think their bikes look "hardcore or badass" in the way that american "biker gangs" view their bikes.
they are no more a recruiting ground for the yakuza than an american street gang is a recruiting ground for the mob. meaning just by being in a bozosoku crew you are not guaranteed a place in the yakuza but if you are ruthless, shrewd, and merciless it certainly gives you an outlet that people would be looking at.