Exactly what pseudochef said. You get a LOT less yeast from secondary, and you're not getting the 'cream of the crop' but rather the 'lazy bastards' that don't want to behave like the majority.
the only reason I'd do this is to obtain less flocculent yeast cells from a very flocculent strain. I'm not sure there's much to benefit from that though.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10