Kevin Cameron wrote a whole column about this, I wish I had a link to it.
He basically explained that rotating mass of a front wheel has to be above an ideal percentage of total bike mass, otherwise it will not have enough steering effect. Cameron's example was a goldwing with lightweight sportbike wheels. Most bikes' stock wheels easily exceed this level, and thus they turn just fine. Lightweight wheels (on a sportbike) reduce the percentage, but its still above the ideal percentage.
I agree 100% that precession does not make the bike turn, precession makes it lean, leaning makes it turn, which is what I said in my 1st post
