that mechanic is either trying to get your business or he's some kind of extreme car buff. 2 to 3 flushes a year is way more than enough. 1 might be fine if you put alot of miles on your car in a year but its recommended that you flush your cooling system every time you change your coolant which is like every 2 years. look in your owners manual, it should tell you the recommended change interval for the coolant.
If you change coolant w/i 2 years you should never need to flush. Modern coolants have very good anti-corrosion packages. I would not contaminate my system w/ a flush unless I was going to totally remove flush by draining the block. This is a bitch to do on 2.5TL's and I just replace my coolant regularly.