It's just information on data they collect over time. In your case I think you're okay as I'm sure MB thought of this given its a staggered set in an AWD and they knew the uneven wear would happen. Most passenger car tires rate anywhere from 9/32 to 11/32 new depending on the type of tire, replacement specs as per the wear bar is 3-2/32... Let's say that the scenario was they saw that your front tires were about 4/32 total and the new measure about 9/32 let's say, well I see why they'd recommend new tires. Now, if the scenario is that your front tires are 7 or even 6/32 and they didn't want to install new tires at 9/32 then that's just plain stupid, you have more than half life still on those. I see more of an issue trying to replace a tire in the same axle with a newer tire being more than 3/32 off. I honestly don't see the issue. Bring me the car and I'll mount the tires, I'll do it and explain this in person