They are obviously flyers (just aswell really) which is always a big tactical plus point. Unfortunately, these flyers only move 10 inches per turn compared to the usual 20 inches.
Their profile is also weak, notably the toughness of 2. Now usually this wouldn't matter too much because swarms are normally just used to provide a blocking unit to allow your army to move more freely while enemy troops are held up. Swarms are usually (actually, I think they are always) unbreakable, this is what makes them so useful. The thing with an undead army is that they are all unbreakable anyway so you already have the troops to perform the blocking tactic.
The flying ability is already provided by fell bats, although you do need to take up a special unit slot to take advantage of this. So, I guess if you wanted to keep your special slots for Grave Guard, Black Knights and Spirit Host then this might be a "slim" reason to take bat swarms.
You cannot use bat swarms as a cheap way to make up your minimum core unit numbers either because, like Dire Wolves and the Corpse Cart, they do not count towards the minimun core unit count.
So, I'm at a bit of a loss as to what else I can say about bat swarms. Please feel free to leave a comment below if you feel I have got this all wrong.
One last thing that holds bat swarms in some small favour in my mind is that I like the models. They have something of a charming comic character about them (in a hideously unnatural undead way of course).
The models are, in my opinion, a nice alternative to fell bats. So, if you already have bat swarms but not fell bats and your enemy is agreeable, why not use your bat swarms as though they are fell bats.
Next up (from their open graves !) Dire Wolves...
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