I leave PureCab on all the time, on 2 or so. It works very well with my current rig, which is the Kemper direct into the FOH mixer, and I'm using open air in ears (very low volume gig). The other day I was tweaking before a performance and had a very strange thing happen; I hit a chord and it sounded like someone cranked the PA up full volume and my guitar was blasting everyone's ears. It was such a strong sensation that I even asked several people if they could hear my guitar; I was in fact muted in everything but my ears. But the ambience in the Kemper, along with PureCab, really made my mind think that the tone was pressurizing the room when in fact it wasn't. It was that reaction where you are turned up too loud, making too much sound in the room, and you instinctively grab the master volume to quickly turn down. I've never had this happen before with any other modeler, not even close.
When an amp modeler sounds so real it makes your gut think you are too loud in the room and not just your ears, you are really on to something.