I found that the Teamwork rules as written were a bit vague on the pool secondary casters would be rolling. Do they actually apply the penalties due to extra factors? If so, then in most cases having a Proxima or Sleepwalker acting as an environment Yantra is actually better (although it does use up an available Yantra).
Maybe this is intentional and Scarlet is right, this is really only meant to be used for the OMG THIS SPELL ABSOLUTELY MUST WORK AND WE ONLY GET ONE SHOT AT IT situations. Mrmdubois is also right, you wouldn't be limited to your own Cabal, and in the kind of situation...