You say yourself that Sigilyph not only counters the standard sets, but works like a charm. Furthermore, your main argument against its inclusion as a counter is that it cannot switch into Stone Edge. What about Xatu? It has poorer stats in every category except attack when compared to Sigilyph while having the same type, and yet there was no argument against it as a counter. The other point you bring up is that Sigilyph must have been previously burned, yet you mention Gliscor as a superior counter that must fulfill the same prerequisites seeing as how it cannot simply switch into a Spore before being poisoned.
... because Xatu wasn't mentioned as a counter in the OP. Why would I argue against Xatu being mentioned as a counter to Breloom, when it wasn't mentioned as a counter?
If you read my earlier post, I comment that I regard Celebi to be a better counter to Breloom than Gliscor specifically because Gliscor needs to activate its Toxic Orb before being able to switch in.
Lastly, a small distinction but an important one: I commented that PS Sigilyph counters the standard Leech Seed Breloom, not the standard Breloom period. Whilst LS Breloom does not run SE, a good portion of the more offensive variants do.
In that sense, your Volcarona analogy is simply not apt - none of Volcarona's standard sets utilise HP ground or rock, whereas 2 of Breloom's 4 sets utilise Stone Edge.
Lastly, the standard Sigilyph does run a Timid nature and has enough speed to outrun a Jolly Breloom
From my experience, most Psycho Shift Sigilyphs run a Bold or Calm nature. The OP of the Sigilyph thread (
http://www.smogon.com/forums/showthread.php?t=79322&page=5) lists Timid as the preferred option over Calm, but similarly does not suggest any speed investment. It's fairly academic, though, and I'll respond specifically to the set you mentioned:
[SET]
name: Cosmic Power
move 1: Cosmic Power
move 2: Stored Power
move 3: Roost
move 4: Psycho Shift
item: Flame Orb
ability: Magic Guard
nature: Timid
evs: 252 HP / 200 Def/ 56 Spe
When I did the calculations I accidentally mixed up the Speed and Defense EVs. My bad. Here they are with the corrected values.
Adamant Breloom Stone Edge vs +1 Sigilyph = 44.25 - 52.29%
Adamant Breloom Stone Edge vs +1 Roosting Sigilyph = 22.12 - 26.15%
Adamant Breloom Facade vs Sigilyph = 46.26 - 54.59%
Adamant Breloom Facade vs +1 Sigilyph = 30.74 - 36.49%
While Sigilyph may not be able to switch directly into a Stone Edge, it can still beat a Breloom with it by PP stalling out Stone Edge and then setting up enough Cosmic Powers to decimate it. Furthermore, the rest of the opponent's team is then royally screwed as Sigilyph is then completely set up firing of powerful Stored Powers.
Problem is that the setting up will never happen against any experienced Breloom player, because they will know to switch out Breloom if they fail to get in the initial Stone Edge or Facade on the switch. They aren't going to engage you in a PP war, and they aren't going to stick around once you start building up your defenses. Assuming your opponent has let you stall out the PP of Stone Edge and build up your Cosmic Powers, yes he is royally screwed, but this has more to do with the fact that he is either a terrible player or has never encountered PS Sigilyph before.
Even in the case where you do not carry Stone Edge, but prefer Facade, if Breloom Facades the incoming Sigilyph, Breloom can afford another Facade before retreating, presenting whatever your opponent has to answer Sigilyph with a free switch in on the inevitable Roost and a ~65% HP Sigilyph to deal with.
I am NOT saying that Sigilyph is a poor Pokemon to switch in to Breloom, but that you depend too highly on your opponent adhering to patterns when playing Breloom to manage the feat. There is no assurance that you will come out on top whatsoever. This is simply not the case with Gliscor and Celebi.
@Supermarth - Thus the distinction then arising between a check and a counter. The 'never ending argument' never occurs when it comes to Gliscor and Celebi, because even if you know they're coming, you can't do anything with Breloom to prepare for it. They 'counter' Breloom by stopping it from doing its job, regardless of how good the Breloom player is. In order to effectively utilise Breloom, the Breloom player must remove them from their opponent's line up first.
Sigilyph does not perform the same function. It stops Breloom from performing its job IF the opponent does not wise up to the tactic, plain and simple.
EDIT: And missed the earlier posts calling for Espeon and Xatu's inclusions as counters. Obviously, I object as well - what on earth is the point of listing something as a counter when it's just as likely to be mangled by the switch in than to actually do something to the Pokemon in question? You might as well list Heatran as a counter to itself, since the standard sets tend to run Choice items, and the thing is immune to its own STAB attack.