Ignoring the response, Agent 13 is reasonably efficient at thwarting over time, clocking in at 6 thwart + block or 8 thwart for 4+card.
The response can be pretty good, though. The list of SHIELD supports is not terribly broad, but there are standouts. Helicarrier in particular refunds Sharon's resource cost over time, bringing her value to "card + time = 8 threat removed." Wow!
For specific heroes, you get solid additional targets: Safe House #29 and Alpha Flight Station are strong card drawing engines. Jefferson Davis lets you go harder on threat removal. And piling up more ammo on Munitions Bunker can enable a massive burst of damage.