I have not worked on RCCL or Celebrity, but I do know that staff members usually DO have a single cabin and it is more likely on a newer ship like Solstice - so don't get discouraged. Guests services is a staff positon (not crew) and most staff members get private cabins. As a stage manager I always had a private cabin.
Maybe the person above is refering to older Royal Caribbean ships.
As far as discounts - YES - you will get huge discounts on snacks, drinks (beer/wine) and other needs (razors) in the crew store. No, you do not get them if you are socializing in public rooms - you pay the same price as the passengers.