Swarm bonus is fluffed as being some gestalt bonus of the combined processing and sensor power. So lots of different perspectives for a Perception test. Lots of different sensor feeds for your social tests to watch body language, lots of processing power to come up with a response. You just need one drone with a Speaker for a verbal communication, not exactly difficult. Not that any sane GM would allow a default negotiation roll from a Swarm of Fly-Spys.
And "Add a Matrix Attribute" Sleaze Rating to one Drone, Attack Rating to another for your Matrix Tests, get a +14 Limit from Swarm. I'd argue that Drones would be able to do a "Snoop" action. That's what at least one of the Drones in Rigger 5 is built for, the Hedge Hog from Page 129.
"..., it can listen in
passively while running decrypting software. Parked in
a hidden location, it can eavesdrop for up to forty-eight
hours on a single charge, gathering data by the gigapulse."
That's the Snoop Action I would think. Although, the Hedge Hog doesn't have a Sleaze attribute by default so, to your point, I don't know how it actually does what it says it does.