A handful of lanterns in Brockholes now have nodes fitted along side the photocells so i'd imagine that the photocells are being kept for backup in case the RCS fails.Normally, the ballasts will operate full-time (ie "dayburning") if a radio control node is not connected at all - hence why my QSM with an intelligent ballasts works. So, I'm not totally sure how a photocell would work in conjunction with that: because if the cell is wired in to control the supply TO the ballast, then the ballast will be dead during daylight hours, and so data from the lantern can't be supplied to the system... and I would have also thought that every night the system would have to re-callibrate itself.
Maybe the photocells are there temporarily until the RCS is fully set up, and then the cells will be wired out, or something...