These two are the comic relief of the novel—the Weasley twins of the Heist Society universe. Angus and Hamish Bagshaw are basically twins, but Hamish is shorter and stockier than his older brother.
If the Weasleys were muggles, we could totally see them doing what the Bagshaws did: steal "a Tyrannosaurus rex from the Museum of Natural History—one bone at a time" (20.38). Pranskters will be pranksters, right?
The Bagshaws are also masters of disguise and pulling off pranks with a purpose. Their expertise in creating smoky explosions plays a key role in the Henley heist, as does their ability to pass for other people—in this case, boiler repairmen.
So while these two are pranksters, they're also reliable. And who wouldn't want two dependable, talented dudes to have their back?