The bikes with mid-frame bags don't need 'em. Keeps the weight central, not over either tire. don't have to worry about balancing the load the same way, and if it's a bike being ridden in woods, no worries about stuff sticking out and bouncing around. I know quite a few dudes using them, and for all day epics where you want to carry mucho extra stuff, but maybe not a larger pack, they're awesome.

Some of the others pictured are cargo bikes - check the wheelbases. They are meant to have big loads on the back end like that, as your body weight is more forward of the load by quite a bit more than it would be on aregular bike with just a loaded rear rack.