I used to use them. Crammed 25k in my pack, one single charge and away it all went. Lovely. I'd done the quest for powder a few times (as opposed to several or above), and ended up with so much of the stuff I doubted I'd ever use it all.
Now I just frequent areas with easy bank access. Well, as it turns out I'd already been doing that, I'd just been using the bags anyway...
Still, it would be nice to be able to venture down into the Fel dungeons without the knowledge I'd be overloaded within a quarter hour.
Non-tamers/summoners can always ride a pack bug around. Those can carry 80k on their own, but granted you can only ride them at walking speed once they've been loaded to a certain point.