It is bad for the engine.

Mercedes, Caterpillar, Detroit Diesel, Volvo, ... all recommend against extended idling hot or cold, only for cool-down when necessary.

Yes it can gum up the rings, will add condensation and fuel to the oil, carbon up the valves and exhaust passages, at least the EGR (shouldn't be) valve isn't open.

Many people will tell you that they've done it for years and the engine still runs, which is not scientific only anectotes. I knew a guy who claimed that oil changes were bad, and that he had never changed the oil and filter on his Ford Fiesta since new, and it still ran fine so that proved his point (?). Mercedes likely has a reason for instructing us otherwise. They really don't burn enough fuel idling to get cylinder temps up, especially if the heater/defroster is on and sapping what heat there is. It's a long warm-up.

Large engines always have a high-idle setting to allow the engine to idle a bit longer, we idle our big DD engines on high-idle often for 5-10 minutes to build air pressure, but that's a bit of a necessity after sitting a while. It probably would be better for your car to either follow the MFGR's recommendation to not idle for warm-up, or try to keep the idle up (1000rpm?) a bit while doing so.

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- Jeff