*can reply again if there's NEW insight to the discussion... well then, here goes*
Briefly at the log from BMB: If frail glass cannon, whether offensive or supporter, is not preferred, then a slow Pokemon is very much possible. Slow mons risk a lot - sweepers that lack Speed take a risk whether setting up or just trading blows (especially if lacking in priority) and will be on the bad end of match ups where both CAP4 and its switch in can KO each other, because of that low Speed. Similarly supporters who are slower risk getting Taunted by a wider group of Pokemon, and thus being caught unable to perform their support function right. Low Speed also suggests it could use Trick Room either to sweep or support its team; as a sweeper, TR setup is riskier than just hitting DD/QD/SS because you're on a timer, and if it's at just the awkward Speed point where it can be run in TR but is 'outslowed' by other common Pokemon (like certain slow walls, especially Ferrothorn running min. Speed for Gyro Ball) making it your Trick Roomer of choice is in itself a teambuilding risk.
There is one more route that CAP4 could take that I hadn't seen mentioned yet, but what sax king above is leading to this idea:
Originally Posted by Fat sax king
I think it's important that there's no universal check to it. It can have its checks and counters, but I think that its checks should depend on the set it is using.
CAP4 could have no universal check if you're not fighting 'it' but its team support. An example of what I mean is SubAgilityPassing Jolteon (or probably more popular now SubCMPassing Espeon because of Magic Guard). Just being able to switch in and KO it won't be enough to make for a 'counter' because if it successfully passes the boost, you'll be stuck in a bad match up against a boosted mon, so a phazer or Taunter must be sent in; but if CAP4 has some offensive ability it could attack it if that choice is made, and retreat (even with dry Baton Pass) to try again later.
If CAP4 were slow too then it'd be riskier to use than other Baton Passers for this (more exposed to Taunt) but by its slow Baton Pass could take the hit for its target and allow something more fragile to enter unharmed. Its success would then largely depend on the surprise factor: CAP4 having many other options than Baton Pass would catch enemies off-guard, but that only works once, and once its set is known it is easier for the opponent to play around, losing much effectiveness. Therefore the player would be taking a 'one shot risk' independently of how fragile CAP4 actually is, or how much damage it ends up taking in practice.
There is also another kind of playstyle that combines support, offensive and risk all at once. If CAP4 was a 'situational trapper' (not one with an ability like Dugtrio, or STAB Pursuit, but one where it has to use a trapping move like Wrap or maybe Spider Web/Mean Look) that can dedicate its set to eliminating its usual counters with the right moves, and thus open up a teammate's sweep, then it's risky (or even suicidal if given, say, Memento) support that influences team building decisions (who needs taking out the most? Will CAP4 be the right lure?) as well as loses much effectiveness if it's discovered by the enemy. And its surprise would again depend on the relative popularity of being a Lure vs its other tactics.