Yes. Being in a queue means you have to constantly keep an ear out for being transferred to an actual human being. Which can be quite hard when the hold music is regularly interrupted with voice announcements. Yes I realise my call is important to you, but if you didn't tell me that every 10 seconds I could stop listening so hard and (maybe?) enjoy the music.
As an aside, their comparison of voluntary queuing with and without virtual queues is suspect as they seem to inform the customer about the length of the queue only when they also have the callback system. These are two separate things and should be measured separately.
Sure, but if the prevailing queue conditions mean I'm forced to wait 20 minutes regardless, I'd much rather do that without a phone to my ear. I'd even prefer a 40 minute virtual queue to a 20 minute physical queue, so it seems appropriate for there to be some discounting of virtual queue times when compared to physical ones.