I partly agree with the original post that it is sometimes useful and fun to shift for yourself. Over the years, I have "played" with my Benz auto transmissions now and then and mostly enjoyed it. I had three 300Ds (both normally aspirated and turbos) and found there were times when choosing my own gear range was advantageous. I find I use it less with my current 92 300E.
But for the most part I have used it to choose a range, not actually shift up through the gears.
For example, in the automatics of certain vintages and certain models (such as my 1979 300D), the start was in second gear, as you mentioned (not your 91, though). So if one wanted a quicker start, one could downshift to L while stopped, and the tranny would shift to first and stay there, even if you shifted back up to D. Then it would run up through 1-2-3-4.
Another time I often use a lower range is local driving. I nearly always start out from my house or a parking lot in S, because that gives just a bit of engine braking (but less so than in the diesels), and it reduces the number of shifts in stop and go driving. I don't move it to D until I am on a road where I know I will get up to 30-35 mph or so and be able to stay there a while. (Note how automatically the shifter will stop at the S position if you just slide it back from park or reverse, and don't make a conscious effort to stop at D!)
Also, if I know I will be on a curving road I will sometimes choose a lower range to avoid shifting up and losing the control of having engine braking and rapid throttle response. Not for aggressive driving, just good control.
And one of the best uses is for descending a mountain, where the use of S or L will save your brakes from overheating. (The W124 series, and maybe others, require you to momentarily downshift to L to get down to 2nd gear, but you can stay there in S once it is selected until you reach a fairly high speed. This is explained in the owner's manual, though not well.)
But you can get into trouble as mentioned. Downshifting when the car's speed will force a rapid increase in RPMs is risky. There was a nice 300SD totaled at a national MBCA event some time ago when the driver downshifted at high speed from D to S. The engine could not accelerate fast enough for the lower gear, and the rear wheels spun, causing the rear end to slide and the car to go off course and crash!
I also met a guy whose family member had supposedly driven his Benz a long distance on a freeway in S, had over-revved the engine and burned it out.
One thing to note is that the transmissions change their behavior in response to pedal pressure. That is, if you hold the pedal down further, the upshift is delayed. This happens even in the older transmissions such as yours that have no electronic controls. And I think the shift points are also affected if you choose a range other than D. That is, if you choose L or S I think the shift points will be at higher RPMs for the same pedal pressure. (But I would not stake much money on that last comment.) So you can somewhat influence the way the tranny shifts for you, even without shifting it yourself.
So I think moderate use of the manual shift provision can be useful if used sensibly.

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DavidB29
1992 300E with ASR
35 years of Diesels until now!