Spiking with Marth is the riskiest of his edgeguarding options. Obviously the rewards are huge, but often Fair, Nair or Bair are better options to keep pressure on the opponent offstage. Dair has huge lag both in air and on the ground, so it can be easily punished if you either miss or space badly and send them in the wrong direction. Ledgehop Dair on very low recovering opponents is an awesome tool though, as it can gimp many characters' Up+B's while avoiding any harm to Marth. Once again, though - screw it up, you may be facing a stagespike.