Another point to summoning the agent version of your selected plot deck could be used as a good distraction so the Overlord can focus primarily on the objective.
My first impressions would be that heroes see the summoned agent as a major threat and would focus-fire on him/her ignoring what they should really be doing in that encounter/quest. Secondly, summoning the agent is a balancing process, if you're using one of the 1x1 base agents, you'd be better off replacing base-like open groups as well. For example, Alric for a Beastman, or Valyndra for a giant or other dragons.
Edited by Coldmoonrising, 24 July 2014 - 06:59 AM.