Hey, I know you own a GT-R and other interesting vehicles, but do you really think you are smarter than the Mercedes engineers of the 1960's??
(this is meant to me a joke and not combative, I hope you agree)
I've attached some pictures of my radio L-bracket which IS installed in my car (sorry).
Without having perfect knowledge, I simply believe Mercedes would not have installed it and they would save money if there was no purpose for it.
And I can see they, without the L bracket, the radio will put quite a load on the dashboard as vertical accelerations (road bumps) act on the radio.
If you don't use the L-bracket, you might, or might not, have bending and cracking at the interface between the dash and the radio.
But, if you do use the L-bracket, your odds are significantly improved and you will probably never get bending or cracking at that interface.
Of course, this is all my opinion and you know what they say about free advice.
Good luck and nice car collection.