Bulky Water types are incredibly easy to switch into, for one. "Ice Beam this, Ice Beam that" but really, how many bulky Water types have you seen blindly use Ice Beam on the switch-in right off the bat? None of them, because that's a dumb idea when they're up against Infernape, Gyarados, or something else they need to stop right away.
Suicune is usually goinna Surf or CM, and bulkier sets will do shit for damage with Ice Beam. Vappy usually doesn't have any room for Ice Beam at all, but even if it did, you can bet it's going to be using Surf or Wish most of the time.
Matter of fact, if you have a Lum Berry, you're free to switch in on almost any wall without giving a shit. After all, what's Celebi gonna do to you? Twave? Grass Knot? Leaf Storm? lol. Set up on its ass.
Also, 76/90/70 are not frail defenses. Even if they aren't amazing on the level of Garchomp, Latias, and friends, they're still a prettier sight than premier OU sweepers like Infernape or Lucario, and unlike Infernape, it actually has a number of useful resistances it can come in on. Either way, I think we can count on it to take a hit or two while it's dishing out punishment.