The way I have heard it, it isn't really such a scam. They have to have the software in the phone to integrate to the hands free features and mailbox, etc built into the car. The phone manufacturers can't guess where the handset will be going, so do they make it so ALL of there handsets will work in any car with handsfree features, etc? Not possible. Although I have heard that if you do have a handset from another vehicle manufacturer, lets say you have a Startac that was sold to you by Audi, BMW or Porsche for example, that the handset may very well work in your Mercedes. A good analogy I can think of is expecting an oil filter for a BMW to fit your Mercedes. They are both oil filters, but not interchangeable, especially when it comes to an "off the shelf" cell phone.
One pitfall to watch out for is when you start changing phone models, say your car was built for either analog only, or a 2 channel only phone (say only TDMA and analog backup, or CDMA and analog backup, and trying to convert to a trimode phone phone (cdma, tdma and analog backup) you can run into trouble.
ps: as far as changing carriers, you very often I believe require a equipment change. If you're only halfway though your calling plan contract and decide to change to a different carrier, quite often the equipment (phone) that you have does need to be replaced, I don't see this as being Mercedes fault. And I'm more than happy to assign fault to Mercedes when it is due. I recently renewed my contract for another 2 years with the same carrier I've had for the last 2 years and even then they required me to replace my phone. I like the old Nokia I had too (5165), The new one I have, a new Nokia, needs the battery recharged about every 3 or 4 days (it's the kind with the lighted side-grip). The 5165 would go 5 to 7 days pretty easily. This things a power sucker!