Why do wireless APs constantly transmit (e.g., with a beacon interval of 10×/sec.), even when no devices are connected to them? Doesn't that just cause needless RFI? Is it really necessary to constantly be sending beacon frames? Is there a way to disable this? (I don't think hiding SSID disables sending beacons.)
Why do wireless APs constantly transmit (e.g., with a beacon interval of 10×/sec.), even when no devices are connected to them?
Just because an AP doesn't have any client devices associated to them, doesn't mean there are no client devices that would be "interested" in associating to them. In particular, this is useful in deployments that have multiple APs as this helps to provide information to clients so they can make decisions about roaming.
Additionally, beacon frames perform a number of different functions in addition to discovery. They serve as a timing source, provide other information to 802.11 devices, allow for power saving features in client devices, etc.
A beacon is required in order for clients to know when a particular AP/SSID is nearby and ready for connections. Without that, the client would never know there's available WiFi nearby, regardless of possibly having previously connected to said SSID. You can disable a beacon by disabling/deleting the associated SSID, or you can simply adjust the beacon rate so it's not 10x/second or whatever yours is set to.
You can read more about the beacon frame and its purpose here