Last post I talked about keeping dice away from NPC mechanics even though they're being worked into the game overall. I still stand by that. I think it makes things a whole lot faster, since all players can roll their dice at once, and the GM can immediately begin moving from player to player comparing his results to the NPC target numbers.
However, I did have an idea for adding dice to encounters overall. It's called an encounter pool, and while it introduces rolls to the GM side of the screen, it's still simpler than rolling for each character the GM controls. An encounter pool belongs to the entire encounter, and it serves as a way to add complications to an encounter.
You can use it to boost NPC efficacy, for sure. Doing it this way is a good way to make your NPCs more effective without shooting their stats through the roof. If you wanted some collectively dangerous but individually weak opposition (Tucker's Kobolds, anyone?), you can apply successes from these rolls to your NPC stats, but they remain fragile.
The encounter pool can be used for more than NPCs however. If you want to establish some difficulties or complications, the encounter pool is a good way to do it. Fighting on the deck of a ship, with waves washing across the deck, can be represented by an encounter pool that makes attacks against all those on deck, for example.