I was told by a mechanic on a BMW I always need to change the rotors when I need a break service, not like in other cars where I could only change the pads and thats it. Its this correct??

no. if you have a shake and your rotors are warped, you CANNOT have them turned. they are warped and no turning can straighten them out. There is however a minimum thickness recamended by BMW and it is usually stamped on the rotor somewhere. I've had a rotor snap on me. it breaks off the hub and free spins and you have no brakes on that particular wheel.

I really doubt it. Although I don't own a beemer, I do the maintenance myself on my jeep. I've change pads before and left the rotors. This was at 60k miles. When I got to 160k miles, I had to change those pads again along with the rotors. What I'm saying is that the rotors lasted about twice as long as the pads. The mechanic is trying to pad the bill. You can do it yourself, it's much much easier than you think.

No you do not, you should change the rotors typically every year and a 1/2, depending on how much you drive it. on my 3 series i put on approx 6500 miles a yr, and change it then. it really depends on your driving habits