I'm amazed that so many motorcycle riders would rather be the "mule" and carry the weight of all that stuff on their back and shoulders, than use a tail or tank bag and let the bike carry it. And I'm not just talking about riders here on these forums, but in general I see so many riders with daypacks these days, mostly in town.
Yesterday, I went for a nice long day ride on the CBR up through the Jemez Mountains, near Los Alamos, NM (yup, that Los Alamos, home of the DOE National Labs). Anyway, I really couldn't imagine carrying everything that I had (in my Ogio tail bag) in a daypack on my back & shoulders for a 200+ mile day of mountain riding... 3L of H2O in a Camelbak Reservoir, 1L of H2O in a Nalgene Bottle, Rain Jacket, Nikon D3100 w/18-55mm lens, 55-200mm lens, plus a bunch of other misc. stuff that must be good for another pound or so.
I do realize that for many shorter riders a tail bag can be problematic when it comes to throwing a leg over the saddle, but a tank bag is a good alternative solution for those who are challenged in the inseam dept. over a day pack.
I do use a regular Camelbak Mule pack w/3 liter Res. for off road riding, but I'm not wearing leather off road and don't carry as much stuff in the pack, just a few small & light items besides the water... heavier things like tools are carried in the fender bag. Still the Camelbak with 3L of water isn't exactly light.
It seems as though the day pack has become a sort of fashion statement for urban street riders... I've noticed that even the squids seemed to have picked up on this trend.