And it's important because a lot of what it can switch into it can learn the move(one of the reasons why it's such a great move, almost every pre gen 6 pokemon gets it)
Here's a complete list of the Pokemon with at least 3% usage that carry Knock Off at least 10% of the time according to last month's 1760 stats:
Bisharp: Hoopa should never be switching in on this thing in the first place.
Scizor: Shouldn't be switching into this either. Still, it can outspeed slower Scizors and do a ton with a Specs Shadow Ball while surviving even a CB Bullet Punch.
Landorus-T: Again, Hoopa wouldn't want to switch into this one either. Kinda like Scizor in that Hoopa can outspeed and KO slower variants with a little prior damage, though.
Thundurus: This is one that Hoopa will actually want to try to take hits from, but only the special versions, which rarely run Knock Off. It's the physical variants that you have to watch out for, which Hoopa should avoid in general.
Azumarill: Again, another physical attacker that Hoopa shouldn't try to take hits from in the first place.
Conkeldurr: Admittedly, Knock Off makes Conkeldurr much riskier to switch into, but it still easily outspeed and KO with Psychic or Psyshock.
Gliscor: There are way better switch-ins to Gliscor than Hoopa. It would probably lose to Gliscor even without the Knock Off weakness due to SubToxic.
Mandibuzz: Mandibuzz makes a name for itself by switching into Ghosts easily, so needless to say, Hoopa doesn't have much business switching into her either. On the bright side, Specs Focus Blast 2HKOs and Specs Thunderbolt has a shot at OHKOing.
Mega Mawile: Physical attacker blah blah blah.
Deoxys-S: Here's one that Hoopa would love to switch into, and Knock Off hurts that (although it's still only used on 1/4 of all Deoxys-S).
Of all of those, most of them are physical attackers or defensive Pokemon that would probably beat Hoopa 1-on-1 even without Knock Off. Deoxys-S is the only one that Hoopa might be able to switch into easily outside of Knock Off. I guess Conkeldurr counts too, but Hoopa still takes a big chunk of damage from Ice Punch or Stone Edge on the switch to begin with.
The two pokemon you named have incredibly stupid(annoying) niches. This thing has nothing going for it. Its immunities aren't a niche. Every ghost has them. What does this guy do that Gengar can't? It is completely outclassed in OU.
Sure, it's outclassed. I won't argue against that. But everybody's acting like it's completely awful, which it isn't. As for things it has over Gengar, it hits a lot harder and doesn't have nearly as bad defenses. Plus, it has STAB Psyshock to blow right past the pink blobs and Sylveon on offensive sets.
Latias also has 110 speed. If Latias had 70 speed it'd be worse than ALtaria.
Lol, even then it would still be way better than Altaria due to its much higher bulk and far greater power. Besides, that wasn't the point. The point is that you can't look at such great special bulk and still complain about how bad Hoopa's defenses are.
EDIT:
@above: Hoopa may have the same special bulk as Latias but lacks literally all of its other qualities: great defensive typing, speed, recovery, and sheer amount of utility options. It also has the same offensive spread as Mewtwo (minus 4 SpA), does that make them comparable when it lacks the rest of Mewtwo's qualities like speed, movepool, versatility, and ways to patch up mediocre defenses on both sides of the spectrum? BST =/= everything.
You're completely missing the point. No one's trying to say that Hoopa is as good as Latias, or even that they're comparable. I'm only using it as an example to demonstrate how good its special bulk is. It's merely response to comments about how bad its bulk is when, in reality, it's actually impressively bulky on the special side.