I'd suggest that stirges are best used as combination monsters, ideally with something that would logically get along with the stirges.
Take something that isn't ordinarily a killer, but that IS a big damage-disher. Say, a medium or large earth elemental (which has no blood and doesn't eat, and thus isn't going to attack or be attacked by the stirges most of the time).
Against a party of five fifth-level people or four sixth-level people, a single large earth elemental isn't a killer. One or two tanks soak damage while everyone else whomps on it from afar, and in a few rounds, the tanks are injured, but the elemental is dead.
When that elemental is accompanied by 8 stirges, though, and the stirges swoop down right into the party's midst, two things happen:
1) Distraction and lost rounds -- people spend time trying to kill the thing with three hit points instead of going after Mister Rocky, which gives Mister Rocky more chances to do those lovely 2d8+7 points of damage per hit.
2) Lower hit points -- if the party's tank has been drained for 8 points of Constitution by a pair of very happy stirges, he's just lost 20 hit points (assuming 5th level). The tank now has a much harder time standing up against that elemental, even if his friends are doing back-rank artillery just as much as usual.
I discovered this by accident in a fight that turned out to be much deadlier than I'd wanted or expected -- 12 stirges plus four hellhounds was very nearly a TPK for a 9th-level party.