Plants actually do benefit from a bit of blue in the flower room- what you may notice if it is strong enough is that the node length during stretch does not go crazy like you will often see under pure HPS; basically, your plants will stay shorter.
Also, vegging under a MH400, a HPS600 won't give you your FULL potential- you would benefit most from about 3x the light in the flower chamber as you had in the veg room, so supplemental lighting is even better!
Tube flouros along the sides of the area where the light is less intense are a good way to arrange this.
Also, keeping a flouro backup light in the bloom room lets you keep photoperiod while killing the HIDs in times of high heat or after a particularly strong hormone application (the instructions on Gravity and others require decreased light intensity after application).