all of the above...sure marketing has something to do with it! But as Greg LeMond use to say, start the season on a steel bike, go to Ti mid season, and get on carbon for the Tour.

Bianchi does make bikes with duel materials...
http://www.bianchi.it/

so here I am I have a steel bike and carbon bike...what do I get next, a ti bike? a steel/carbon bike? a alu/carbon or ti/carbon bike?