Ah, the humble bat swarm, one of the most rarely used units in the vampire counts army, and with good reason, for they have little use in any list I can think of.
Redirectors, you say! Well, at 35 points each, I'd happily take one spirit host (65pts) or four fell bats (80pts) for two bat swarms (70pts) any day. But, Mr Saturday, they're fast, you holler in hoarse desperation. True, they have Swiftstride, but again, our spirit host is as fast on average with movement 6, and the fell bats, they fly as well. Aha, you thunder, triumph in your eyes, they can hunt war machines. Weeelll....technically, yes. However, with strength and toughness 2, they may hold up a machine for a turn or two, but I doubt they'll destroy it. They may pass on some nasty disease to the crew, certainly a painful scratch or two, but that won't help your vampire lord with the cannon ball tombstone. At least they use up core points, you scream, tears rolling down your face. Nope, sorry, they don't count toward core points.
Now, pull yourself together, and we'll go through the motions. Here's the stats for our ineffectual little critters:
M WS BS S T W I A LD
1 3 0 2 2 4 1 4 3
Yes. Rah-hu-ba-hish. They are also a swarm, but as undead don't use the squish rules, using unstable instead. Now, again, the main value of these guys would be as redirectors and general irritation. Yes, they have 4 wounds, the fell bats have only 2. But you almost get two fellbats per base of bats. As for spirit hosts, they are almost two bats per base of spirit host, and they are ETHEREAL, which will stop anything short of a magic cannonball. So, in the support unit stakes, the bats come out poorly. So poor, in fact, that there really is no need to elaborate much further. The only real use I can see for bats are screening. They can soak up more damage than a fellbat, and you get two (almost) for one spirit host, so they can cover more space, BUT, with true line of sight this is of dubious value against shooting.
So, a big thumbs down from the vampire general. Next up, the corpse cart.