Well, it
would be a great tool to have!
In fact I just bought the software and the cable to do this on my VW. The problem with doing to on a Mercedes is the software is 4-5 times as expensive. There is generic OBD2 software available for cheap, but to scan the transmission, radio, etc. the only decent tool I'd found is called
Carsoft. Last year they quoted a price of nearly $900; this is the "impractical" part.
Still, if you plan on keeping a Mercedes or two for a long time, it seems it would be worth the investment. I'll play with the VW first, and probably won't get the software until an error code appears.
How many dealer visits would you have to avoid to cover the $900 cost, I wonder?