You can read about many opinions regarding prolonged idling of MB diesel cars in this old thread:

http://www.peachparts.com/shopforum/showthread.php?threadid=84120&perpage=15&highlight=idling%20diesel&pagenumber=1

My belief is that idling MB diesel cars is not a good thing. However, if you intend on keeping it idling for very long periods of time, you must change engine oil more often to remove harmful soot, and do an Italian tune-up frequently to blow out the excessive carbon deposits due to prolonged idling:

Here's my post about MB diesel idling taken from that old thread.

"If you intend on letting your diesel idle for such long periods of time, you need to change your engine oil much more frequently because idling a diesel creates LOTS of soot in the engine oil, and lots of carbon deposits in precombustion chambers and around glow plugs and injector nozzles. MB diesel engines can only tolerate about 2% soot (remember that the original oil filter design on these engines does NOT attempt to filter soot at all).

Carbon deposits will greatly reduce the heating performance of glow plugs as well as shorten their lifespan, and probably affect injection jets.

Do not compare your MB diesel with big rigs and diesel locomotives. MB diesel engines use Indirect ignition while the others use Direct ignition. Big rigs and diesel locomotives have very large oil capacity, thus it can handle a lot more soot than your MB diesel.

Indirect ignition resorts to precombustion chambers to handle a much weaker injection pressure while direct ignition employs much more powerful injection pressure to blast diesel into the regular (big) combustion chambers. The tiny precombustion chambers are highly susceptible to carbon deposits because of its space constraint. You may want to do an "Italian tune-up" from time to time to clear up the carbon deposits due to excessive idling.

IMHO, you should not idle your MB diesel for such prolonged intervals. 5 to 30 minutes idling at a time should be max."