Promiscuous mode in a wired network still requires you to physically connect to the network. At best you can use a passive EM detector to see all the traffic on a set of wires, so you don't have to cut them temporarily, but that still requires physical contact, and can only see things which are routed down that cable. For any business that guards its doors and ethernet ports, and has routers/switches instead of hubs, this is pretty much complete security.

For wireless, you... sit up to a few hundred feet away (miles if you have a good parabolic antenna), and run Wireshark. It's entirely passive and undetectable.