I use footstooling extremely often. Vs. King D3 when he is trying to jump to the ledge. If he only had one or two jumps left, then footstooling him forces him to upB with minimal mix-up options, resulting in the ability to edgeguard him further for massive damage. Sometimes I do this to Kirby too.
Vs. Lucario, when he clings to the wall out of UpB when I'm hogging the edge, I footstool him, and he falls down hopelessly, because it doesn't replenish his UpB. This is incredibly easy to execute on Yoshi's.
Vs. MK, and Pit (Charizard doesn't really try this), when he is gliding low to go underneath the stage, I drop from the edge and FOOTSTOOL THEM OUT OF GLIDE.
IN DOUBLES, WHEN MY PARTNER HOGS THE LEDGE, YOU CAN RUN OFF AND FOOTSTOOL THEM WHILE THEY ARE HANGING ON THE LEDGE. It essentially creates a much better edgeguard, as you get nearly double the height than if you were to jump off the ground, and still retain your second jump.
In doubles, I footstool my partner to give them mobility back while I can recover safely and guard them from danger. This is assuming my partner has a good recover (G&W, Snake, D3, Wario, and MK all can recover from a footstool reliably).
I have many more uses for footstool, this is a widely underused tactic.