I guess USB A is still quite common in things like docking stations etc., where you would typically use such a receiver.It's bizarre that even the new Bolt dongle is still USB-A
Btw., one thing to note especially for Mac users is that I'm not sure if the Bluetooth connection will work during boot on a FileVault-enabled system. At least if I remember correctly this was an issue on Intel system - you only had a Bluetooth connection once booted up.
With the receivers, this is not a problem, as they basically look like a USB keyboard to the system.
Edit: previous poster just beat me to it