more weight.... and more fuel to burn.
as far as i know, you have to pay by weight (or categories of that), so apart of fuel consumption airlines want to reduce because of that as well, i suppose.
anyway, i think it's not by accident that they want reduce weight... yep, maybe it's because less dry operating weight -> more load, but i really feel it's for fuel saving.
z
p.s.: i did a small research now, and it seems, according to several forums and articles, that fuel consumption expectations met the practice, after months of operation! cool.