John, here's the skinny as I understand it.
There's no inherent, obvious advantage or disadvantage to a well-made multi-material frame. There's only the potential to convey the properties of each of the materials employed -- Lemond, for instance, will claim that its 2004 steel/carbon Zurich sports steel's silky, responsive ride while saving weight and gaining stiffness via its carbon upper-half.
How much stock you put in that assesment, of course, depends on how much stock you put in those relative properties to begin with. Most people, myself included, put some faith in the relative properties of different materials, and likewise find multi frames exciting.
The benefit or disadvantage of a multi-material frame is likely to be as subtle and personal as the benefit of say, ti over aluminum, or vice versa. The only undeniable benefit is that they can be intriguing and cool, which is important. Sort of.