Gem,
I agree with a lot of what you say and yes they are supposed to go to specific seats.
A lot of what you say is true and these customer surveys are not a good or accurate way of assessing individuals.
They do show trends and they are more than just about the cabin crew. They talk about food, seating, check-in etc. so they can flag up specific problems on specific routes.
Staff assessments are always hard and just as it is not good practice to say everyone is good, it is also not good practice to encourage people to assess their staff, making the assumption that a certain percentage must be below standard either.
True and valuable assessments are what is required and when someone is good it should be ok to say so. Many companies put their managers under a lot of pressure to find a fault in someone or something that needs improvement, whether it exists or not and this actually can lead to demotivated staff because they wonder what will ever make their employer happy!
So good and fair appraisals are as you say important and even more so if your pay depends on it!!.