50% of max heart rate for an hour? It seems inefficient and not stressful enough for my taste (not to mention boring), but maybe as a once-weekly thing on a day following a strenuous weight session it might be warranted as a form of "active recovery." Or, of course, for beginner exercisers, anorectics, the elderly, or the terminally-sedentary.
At the same time, moving at all is probably a better idea than not moving. It burns no fat (not that you need that) and builds no strength, but certainly helps maintain some minimal degree of endurance and mobility.
I tried to introduce one hour of this sort of thing into my weekly regimen a few months ago as a "recovery day" (with a mix of treadmill, stairmaster, and elliptical) but I just can't stay with it. My body seems to insist on bursting into one-minute sprints in the course of it every several minutes, thus converting it to HIIT. Maybe it's because I am in half-decent shape, but my body just wants to pound it every few minutes instead of slogging along. I guess that's a good effect of my fitness training - energy.