Of course it's logical to compare cars even in an official thread dedicated to a specific model. There's no outright rule that forbids this.
However there is a long-standing, unwritten rule in the culture of this forum that certain behaviour (unbridled fanboyism) is frowned upon as it reduces the quality of discussion in threads where reasonability and objectivity are valued by the majority of members in our community.
Experience shows that some people lack maturity and self control to the point that reasonable discussion is lost to an imaginary pissing context. Such rhetoric is discouraged in the official model threads and members are advised to take it to the Internal Combustion section of the forum where a more robust "gloves-off" level of discussion is acceptable:
Welcome to Internal Combustion, where you can discuss car competitions, compare specs, and find test results for your favorite vehicles.
www.germancarforum.com
Some people just haven't got the intellect and maturity to realise that, no matter what, they're just not as invested in a car brand as they imagine they are.
Edit:
@leylandi - the forum has kindly provided you with a thread to continue with your pro-Mercedes, anti-BMW propoganda.
Continue with your squabble over there.
Continue being a nuisance on this forum and it's not only threads that you will be banned from.
@Uygar - please be the better person and engage leylandi in the appropriate forum sections.