If I remember correctly, the dev team removed insta-cast of BeneII because when you queue it after BeneI it wouldn't proc in time to use it as instant. Like this:

Charge BeneI -> Cast BeneI -> Queue BeneII -> Charge BeneII -> BeneII procs -> Cast BeneII -> proc goes with the cast.

It was frustrating and they removed it...

They also said something similar was happening to Fire/FireIII, but they said they made adjustments to it. I think it works fine, but I couldn't say for sure since I don't usually queue Fire->FireIII, I usually queue Fire -> FireIV...