I think this is where much of the vitriol lies - the underlying message bandied about is that the royals are the greatest thing since sliced bread, and then when you couple it with the notion that they are hardworking, I think that most of the vitriol against this couple is really directed at their fan base rather than them. Obviously their roles in this world are different than the average pencil pusher - but the celebration of these two as some sort of demi-gods is what starts the backlash.
Obviously, Kate cannot work at the moment, but during those times when she could, she hardly did.
As for the fact that the Firm did not expect the Cambridges to take on a more royal role, I look at the evidence of the readying of KP, then we suddenly had the gap year, then suddenly Will was back to flying. I don't think this was the Firm - I think this was Will. I am not judging Will's decision, but the idea that the Queen and Charles are completely on board with Will's ideas or that the Queen rules the roost is belied by the events of the last 14 months. Even George's christening was a small, subdued affair when compared to other family gatherings - which the Queen is said to love.