RSR, it's hard to say but on past MB's that I've had I can get about 2 brake jobs in between needing rotors. Sometimes the price on the rotors is so cheap and labor rates are so high that they figure that it's a better deal to change the rotors. Also it may just be they way they do an estimate just in case it does need rotors.
If you take your wheel off and measure the thickness with a micrometer or calipers you can see for yourself. A place that sells pads should be able to lookup the minimum thickness and most of the time it's stamped on the back of the rotor. One way to call their bluff is to ask for the old parts back and if the rotors still have a lot of life in them have them take back the new ones and put the old ones back or credit you the difference.
Truelove, if you want pads that last and produce the lowest amount of dust it's going to cost ya. The Porterfields are the only ones on the forum that people are consistantly happy with. Check my website for a link to a place that sells them at a discount
www.Buellwinkle.com