I do recall hearing of someone running with a pack of spiders (I don’t recall one) during the invasions and having a grand time.
Might have been us, haha! We did have a blast using 2 sets of Wolf Spiders following up our tanking Lion. As Pawain and Railshot said, great option when mobs don't retarget or have area spells. In the invasion they died VERY quickly to Stratos's area lightning attack. After having such a success on the invasion generals we tried them in the Blackthorn Dungeon mini-cities... One word: FIASCO! Those mobs retarget all the time and one-shot-kill, can't be compared!
If I'm not mistaken they have good poison and magery too. But they only have around 180-200 HP. Seems like they would die a lot.
They don't have magery, but when boosted by the Provo-Bard Songs they did around 90-110 damage each bite, and they are FAST. We still use them occasionally, they do die a lot but speed up killing things as well. The tricky bit is often they tend to wander off on their own, and without the pack instinct they become useless. Even in the invasion they sometimes stubbornly ran off to attack an own target. Then they are annoying, but overall we enjoy them a lot. Hmmm, tempted to see how they will do on the roof. Might try them this evening.
Edit: Did have a run with the spiders, started with Juo'nar and didn't do too well, 3 died. But at least 2 felt preventable. Healing without protection would help. And he kept targeting the spiders, whereas on previous runs he usually mostly keeps targeting our tanking beetle. The others were quite doable, although on Anon we kept having to stop them as he went Earthy form very often... Might say they are TOO good for that heh. Virtuebane finally went down very fast (we don't have a cold pet in our set-up so normally takes long), and even his burn to ash didn't kill the spideys. Did call them back for the next few burn-to-ashes, with the warning that is doable. On Virtuebane they averaged 20 damage per hit, so 100 per round if they all hit. Much better than the 16 of our Fire Drake that they replaced this time. End conclusion: well doable, but not ideal.