IMO, if you're not bouncing off the marker/box/bench, and you're not pausing on the marker/box/bench, there are few reasons to not employ a depth marker to your squats. I feel that the pros outweigh the cons.
To me, the major con is that you're SLIGHTLY relying on an outside source to reverse the motion at the bottom of the lift. I say slightly, because as long as you're controlling your descent, not pausing on the box, and not letting the point of contact alter your form, the depth marker is doing very very little to help the actual lifting itself.
Anyone have a view on this?