Well there's lots of examples, like with common Volt Turning teams with spikes. Darm comes in on your Roserade or whatever. You don't want to risk it so you switch into Suicune. With Flare Blitz and some hazards you're down 40%. Your opponent then switches into Shaymin. If you were Milotic or something you could Recover off the damage on the switch then be at full for the next assault. If you were Slowbro you could Slack Off or even just switch out and let Regenerator patch you up. Even if you Rest up on the switch being asleep sucks.
Same scenario comes around again and you bring in your non rested Suicune on the Flare Blitz. You get 2HKOd.
Same scenario with rested Suicune, you get unlucky with Sleep Talk, pulling a Rest then a Calm Mind. This will happen 4/9 times if my math is correct, nearly half. In this case, you get 3HKO'd without even touching the Darm whereas even Milotic would have been fine. I've had this kind of thing happen while using Suicune. They don't even have to stay in Flare Blitzing, they can bring their grass or electric or whatever and not got hit almost 70% of the time.
Even if you Calm Mind on the switch to their Shaymin or Zapdos or whatever you're still doing crap damage while they smash you.
252 SpA Life Orb Shaymin Seed Flare vs. +1 252 HP / 4 SpD Suicune: 252-299 (62.37 - 74%) -- guaranteed 2HKO
+1 0 SpA Suicune Scald vs. 0 HP / 4 SpD Shaymin: 59-69 (17.3 - 20.23%) -- possible 5HKO
I'm not saying that Suicune isn't good but it has several serious flaws. Mono water isn't the greatest with Kingdra and the like running around, not even mentioning stuff like Poliwrath. Even in the case of Milotic you mentioned, Suicune isn't winning as Milotic generally carries Dragon Tail or Haze. Slowbro and even Milotic can hit their counters with strong coverage moves, status, phazing, a whole bunch of things since they have a moveslot to spare.
Please read this again: ''It doenst really get worned down since it only has to come in once anyway''. Once. Crocune is not to be used like a regular bulk water. Crocune purpose is find an opportunity to setup once its checks are weakned or removed and then seal the game. Stop comparing it to slowbro and milotic. Slowbro and milotic use their bulk to switch into powerful moves and their instant recovery to keep themselves alive through the match. Crocune will use its bulk to tank hits while setupping once the path is clear. Again your argument about shaymin/zapdos/w/e coming in and forcing you out is downright irrelevant since they NEED to be removed in order to sweep.