Personally, I've never found the need for more than 1 slot. Also, I always live with any changes I make with Dirac for a few days before deciding if it's a calibration or curve design that I like. I found quick A/Bing to be ineffective in my room.
I would guess that Arcam as an early adopter of Dirac probably didn't think people would need more than 1 slot when they designed the current range. I would expect they will include more slots in future designs just to be competitive.
In my application I have a small room and an extreme recliner sofa which can move the listener quite a bit further away from the speakers / towards the back room, hence why I'd like an extra slot - "Sit up and beg" vs "loungin'" . At the moment I tend to DIRAC somewhere between the two.