The energy regeneration from Limbo's passive while in the rift is 1 Energy/second. This applies to everyone in the rift, including squadmates that you have banished. Also, remember that abilites cross from the Rift plane through to the non-Rift plane. My squad and I exploit this frequently. I banish my squadmate, and then they activate their ability. I do not recommend doing this to someone that you aren't in a squad with before the mission starts. Most people get annoyed by Limbos, they think that everyone is trolling if they play them. However he is a ridiculously powerful WF.