Those cars were developed for a specific mercedes competition. The fact they are available in game is just a bonus. You can still race them as a one make series? Just because it isn't "real" doesn't mean there is no merit. None of it is real after all....it's just computer games. You wouldn't have a clue if any in game car is a real version of the car anyway. Clue: none of them are, they are merely representations based on the limitations of the game physics. Eg if S3 had done the 3d model of the 90 V8 as a Jordan, and called it a Jordan and everything looked perfect but the underlying physics was the same you would have no clue it wasn't a Jordan.
Don't forget ACC in that list. As the name suggests, it's all about the experience with RR. I am drawn to RR because of the driving experience it gives me, wheel to wheel racing in this Sim is fantastic. You can throw as much money as you like at real world licensing, you can add the eye candy, rain, puddles, moonlight etc etc but if it's not exhilarating, if it doesn't make the hair on your arms stand up, what's the point? Beauty is only skin deep, what lies beneath is the most important thing and with RR we have all watched as this Sim has been fettled into what we now have, with plenty more to come, AC & AMS have moved onto new versions, rF2 are still promising a new UI, iRansom, enough said, ACC, one series, couple handful of tracks, pretty but just doesn't rock my boat. RR has found its own path in a niche market and for me, gives the rush that the racer in me craves, whether I'm racing Kimi or Lemmy the excitement is the same. Pretty will get you in the door but if you lack personality, you'll be ushered out the backdoor before you got to the bar. All the Supermodels I knock about with are as dull as ditch water as a rule!
I like those 3 cars, they are very nice to drive. I also would like to have those complete DTM series, but if this is not possible for whatever reason, i'm very happy to have at least those 3. If you are not happy with those 3, don't buy them. What's the point of having 1 Ferrari F1 car of a certain year with only 1 livery in AC ? What's the point of having the BMW and Merc DTM 1991(2) and not having the Audi and Opel in AC ? I can make a very long list like this about almost any sim, but what's the point ? Conclusion : take it or leave it, buy them or not buy them.
I did a scientific test, found out two things, walls at Monaco would be bit too close for my driving lines and in some other games tires become soapy in just two laps or so it seems. I did win still even 2nd place AI must of been Schumacher Unless you are 1st to 1st corner, racing there is very uninspiring, even if you would be 2 seconds faster, there really is no places to overtake, but for hotlapping it could provide value, however any track added would need to provide also online race kind of fun and crawling in queue line is what seem to happen quite a bit with any more than few cars, it is just is pretty hard to see such track in this game.