There's a reason why Apple doesn't have Handoff enabled on capable macs: Encryption.
The 2011 MacBook Air and 2011 Mac Mini has Bluetooth 4.0 LE, but doesn't have the proper chip to help encrypt the handoff signals to limit it to matching devices with matching Apple ID. So while Handoff may work with the enabler, the signal is not encrypted and is very open to attacks.
Without the proper encryption authentication, there are high chance that some non-matching iOS devices nearby may be able to pick up the handoff signal. That can mean disaster in terms of privacy.
(Imagine you were searching XXX content on your mac, but you decided that you may be comfortable watching it on your phone, so you hand it off to your phone. But, since that mac is sending unencrypted handoff signal, junior using his iPad may accidentally see your XXX content if bluetooth is turned on and has the compatible iOS device)
Basically, for handoff to work safely, Bluetooth LE signal has to be encrypted, and Older Macs and USB Bluetooth Dongles don't have proper chip to encrypt to signals
(note that encryption chip I'm talking about is proprietary)