The disadvantage is that I don't see how you'd 'drain' it over the bag without aerating the hell out of it. Aeration = oxidation = stale beer flavors rapidly developing in the bottle.
I.e., no shelf life for your sweet nectar.
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