And if Breloom is behind a sub, then what? What you just said applies to Mence as well. It can be KOd, but requires a different method from the norm. And apparently, no one is willing to stray from the norm, which is why everyone wants him and DNite banned. Because they require using Pokemon besides the standard Pert, Scizor, etc. is why nobody wants them around.
Breloom should never get a sub up unless you were switching to something to counter it. Otherwise you just break the sub--what else would you be doing, letting it set up? Intelligent players using a set with Substitute on it (the most dangerous of these is the SubSeeder) will Substitute first, knowing that this eases prediction for Spore and also knowing that you don't want to waste your sleep on, say, a Clefable, when you can just Sub as it switches in, seed it or punch it, and watch it die. You can take advantage of this by switching in something with Taunt or Encore the first turn you see Breloom. On the off chance that they Spored, yes, something is asleep, but now they can't sleep anything else and they're dead. At least ONE other Pokemon on your team must have a Fire, Ice, Flying, or Psychic move, right? If they played it safe and Subbed, you can Taunt it or Encore it (thus forcing it to leave).
Alternately, you can switch in something faster than Breloom which knows Substitute. Your own Substitute blocks everything on Breloom that's not an attacking move (i.e. Spore and Leech Seed), meaning that it can't do anything to you until its Sub is broken one turn later. Once the Sub is down you're faster and you win. If it attacks, thus breaking your sub, you attack back and take the hit. Hopefully whatever you switched in isn't weak to Focus Punch.
Anything faster than Breloom with a Lum Berry that can OHKO beats it, because either it Subs on the switch or it Spores on the switch. If it Spored on the switch, it has no sub, and you can OHKO next turn. If it Subbed, you can break the Sub while it Spores to no effect and OHKO the next turn.
Again, you could just take the sleep.
The thing that makes Breloom so dangerous is that people don't think the right way when they try to counter it. Too many people think like this:
Dude just let Breloom sleep a status absorber/sleeptalker then lol at it with moltres, salamence or skarmory.
So many times I've seen someone switch in a Starmie when I have a Sub up, watch me Spore, and switch out to their counter as I put up a sub. Well, great, now I can sleep your main counter to Breloom because Natural Cure prevented the sleep clause from stopping me. Thanks.
As long as you think about Breloom the right way it's not terribly dangerous. The problem is that hardly anyone does, and thus it is one of the most dangerous Pokemon in the metagame.