First off, it certainly won't hurt. But do you need it?
Will it make your brakes last longer?
I would love to hear that explaination. Based on the fact that they say it will get rid of rust, says that person is a salesman more than anything. If it is so bad that there is rust in the system, there would be some serious problems. Which I highly doubt are present.
What brakes service are they doing and how much to flush?