Planking isn't really stalling (IDC, sonic's homing missle thing under the stage is) You opponent can easily run over to you while you are planking and grab the edge to edgehog you, or stage spike you with an aerial.
You also have to think about it from a different perspective. Take marth, for example. With his fast disjointed hitboxes, he can make it a nightmare for his opponent to return from the ledge back on to the stage. If marth spaces properly, he can cover: normal getup (with fsmash, ftilt), attackup (with counter, fsmash, ftilt), roll (with anything...), jump (with fair and nair), and most ledgehop aerial options, since marth's disjointed hitbox outprioritizes everything.
That's forcing the person on the ledge (who is already in a bad position) to return to the stage, but just letting your opponent to return to the stage wouldn't work either (this is a fighting game).
I can see arguments for both sides, but since this isn't some broken tactic that makes you invincible, it shouldn'd be banned. Yes, it puts you at an advantage, but that's the point of any game, to put yourself at an advantage and you opponent at a disadvantage.