Why have it in the game? It is because it is history, and you should be able to learn about cars then and now. This game is about the first racing game where you can race the very first patent automobile. It is in the game so you can play a role in history. The only old-schooler I don't have yet is the Ford Model T. The ONLY way to get it is to clear all Super License tests with Gold (even the ultra-long Nurburgring Nordschleife). I have a feeling that in terms of automobiles older than almost 100 years ago (as of this date, anything older than April 17th, 1905), the Model T seems to be one of the coolest vintage cars, perhaps with great performance even it its time.
To help you appreciate seeing these Daimler machines in the game, let me ask you an alternative question. Why do you think throwback jerseys and retro clothes are making a comeback in current fashion? Times then are different. In much the same way as you're able to compare a Skyline R30 to the R34, same can be said about automobiles past and present. Europe invented the car, and since PD wanted to make GT4 a celebration of the automobile past and present, these two Daimlers have every right to be in this game. Not every machine has to be a speed demon. That can be said about several cars. Let me name a few while I'm at it... Scion xB, Honda Beat, Subaru 360, Fiat 500R, Daihatsu Midget D-Type... just to name a few. I can tell you that every last automobile on Earth wouldn't have been as successful as they are now, had it not been for both of the Daimlers shown in the game. Had the Wright Brothers not invent that first airplane, flight would have been nearly impossible. Then, carbon fiber wouldn't have been used for cars since some planes used carbon fiber. So in other words, don't you want to know how cars used to be? Then win both machines, go into Arcade Mode or do a Practice in Gran Turismo mode and see what it was like then compared to now. Only so few can understand and appreciate what these two Daimlers mean to automobile history.