I can't see how people can contradict themselves so clearly, but perhaps I explained it poorly.
If MK DownB and you shield, he can not attack, and safely assume any location in his radius. If you do NOT shield, you are susceptible to the attack. Therefore, logic seems to suggest that if you do not hit MK out of the start up of this move, you should probably shield. It is not really viable to use an attack near you, because then YOU are the one using a laggy attack to punish something that MIGHT happen, and leaving yourself open, especially if I just teleported a dsmash distance behind you.
What this means is, since DownB frequently causes people to shield, and it's hardly punishable OOS unless you land in grab range like a moron, it basically allows you to move to a more favorable position. You also get additional movement after turning visable, which is WONDERFUL for baiting and escapes (seriously, teleporting towards your opponent, and sliding backwards on reappearance is LOVELY)
So, really simple.
DownB = your opponent shields
opponent's shield = DownB is safe
Opponent doesn't shield = Opponent is not safe.
Therefore, while using DownB does have a risk element, it's actually in MKs favor when used properly. For those of you that use Ftilt to stuff dash in approaches (and get shielded and punished) or use Dsmash when your opponent can shield instead of punishment (which is a lot of you, and sometimes me too), don't give me crap about stuff not being safe. I know you play MK, and you're used to being safe 80% of the time. However, the risk vs. reward for this move is in your favor, especially if you use it primarily to escape dangerous situations, or create openings, without frequently risking the laggy attack.