I'm in Europe, so some things are different, e.g. low sulphur diesel has been normal here for quite a while...

1. I can remember my father had a metal funnel with a "won't pass water" filter screen, he used to use it a lot, and ALWAYS used it on any fuel going into a boat, of course he came from an era where there were not standard fuel handling methods.
2. In my own experience, fuel contamination varies a LOT, I don't just mean country to country or season to season, I also don't mean winter vs summer fuel, I mean THE SAME FUEL, for example one local bunker / distribution centre was built on a flood plain, a neighbour was foreman on the build, he said they had a hell of a job because of the water table, ever since fuel from that site that was stored in below ground tanks has been "iffy", how iffy depends on how high the water table was...
3. IMHO we reached a high point in fuel handling "care" in the eighties, since then everyone has been cutting corners and saving money, especially fuel stations, here in the UK for example a fuel delivery pump can deliver up to between 10ml (1%) extra and 5ml (0.5%) less per indicated litre of fuel delivered, all the money goes on systems that will get that as close as possible to 4ml less, e.g. 996 cc of fuel per 1,000 cc indicated on the pump, any filtration that exists is solely there to protect that metering equipment.
4. Both boats and cars cross international borders here like yanks cross state lines, except an international border means different fuel regulations and practices, MB diesels can expect to fuel up all over the shop, even African fuel, so their standard fuel filtration is better than many brands, and notably better than many US makes.
5. Back when I worked in the private luxury yacht business this was especially notable, go into the engine room on a yank built yacht and you might see beautiful CATs or more commonly Detroits, and a piss poor fuel management system. Go below on a EU built boat and you'd see MTU's and most importantly a decent fuel filtration system AND an Alfa-laval.
6. Alfa-laval is basically a centrifuge, it doesn't just separate out water and other contaminants, it also settles out a hell of a lot or particulate matter, standard practice when bunkering (fuelling) was several separate bunkers and a day tank, say 1,2,3,4,5 & 6 bunkers plus day tank. Engines were fed from the day tank, day tank was fed from 1 & 2, bunkering filled 3 to 6, then the alfa-laval was connected (supply and return) to each newly filled bunker and run for several hours until it stopped producing ****, then move on to the next tank.
7. In addition you had tank > water trap > primary filter > secondary filter and this was always a double / redundant system so you could swap out filters while the engines were running.
8. One of the common problems / misconceptions was the fact that diesel contains living bugs, this was especially a problem for boats fitted with stainless (300 series) / inox fuel tanks, because the bugs used to eat at the welds, I've poked oxy acetylene nozzle cleaners several mm into these "boreholes" and they often lead outside the tank causing seepage / leaks... only adding a UV stage to the alfa-laval would kill these bugs.
9. One of the other common problems was water, either pumped in with the fuel (see #2) or as condensation inside the tank, we used to have a pink paste on a dipstick to test for this, again alfa laval was the cure, cheaper than changing Racor filters every day.
10. Getting back to MB, yeah, they have some of the best standard fuel systems going (US market products may be different in this as well, I do not know) but I shake my head in despair every time I hear someone who bought an old Benz just to run it on WVO etc, what the hell are they thinking? Like buying a Rolex to go hard rock mining... yeah, it'll handle it better than a Timex... that's not the point...
11. I run a 1990 W124 E300d, this has the straight six 3 litre non turbo 603.912 engine, a fantastic engine, I don't know if it was made / available in the states, totally non electronic / gizmo, City = 30.37 mpg, 90 kph / 56.25 mph = 45.56 mpg, 120 kph / 75 mph = 35.76 mpg (NB Imperial gallons) and will cruise at 120 mph.
12. I have always been anal about filters, both fuel and oil, Fiam (dunno if you get them there) you couldn't pay me to fit to a lawnmower, for the Merc I buy Mann OEM filters.
13. I do not run a common rail diesel engine, or an ECU controlled diesel engine, you could not pay me to run either.
14. When changing the fuel filter on a diesel engine I have always filled (primed) the filter with ATF.
15. When filling the tank on a diesel car I have always added an aerosol car cap/lid's worth of ATF every other fill up.
16. 200 miles before doing an (lube) oil filter change I have always added a litre of ATF to the lube oil (my current MB when I bought it had two noisy hydraulic tappets, no more) and let it clean the engine.
17. I have always done the (lube) oil change PROPERLY, e.g. drain the bloody sump, none of this suck it through the dipstick pipe rubbish.

Bottom line here, the standard MB equipment is quite good enough, if fuel in the US is as dirty / contaminated as it sounds fro this thread, then I would be buying a "won't pass water" filter funnel and stowing it in the boot/trunk, and every time I went to a filling station NOTHING would get into my fuel tank without first sticking the "won't pass water" filter funnel in the car filler tube... yes, it will slow down the fill, **** them if they can't be bothered to sell you decent fuel, and you will SEE the proof in the funnel every time, and so will anyone else who looks.

BTW pump diesel here is now UK£116.5 per litre, that's US$1.85 per litre, or US$6.99 per US Gallon.

I still wouldn't run WVO etc in my 20 year old Benz...