The idea is to stop putting the current through the receiver for the high drain servos. The digital servos use a lot more current than standard servos. A lot of the receivers were designed before digital servos were readily available and may struggle when the extra loads are being placed on them.
I am just being extra cautious. I have not experienced any problems but I am trying to pre-empt them.
The only other time I have used two battery packs is on my Kyosho Corsair (
http://www.mjp.co.uk/model%20aircraf...air/index.html ) where I have a separate pack for the retract servo. This is incase the retracts foul when in the up position and cause the retract servo to stall and drain the receiver pack in flight. This way I would only loose the use of the retracts opposed to control of the model.