E46_Driver said:
Sedans are nice and convenient for backseat passengers, children, pets, groceries etc. Try and remain objective. BMW makes a "sprorty" sedan. This is not a sports car. The M3 is a two door Coupe for a reason. A sports car by definition is a two door. Does Porsche, Ferrari, Lotus or Aston Martin offer a four door sedan? The answer is no. We can agree to disagree knowing most people understand a sports car to be a two door coupe
I'll agree only to a part - sports cars are two doors (and most often 2 seats only) are for obvious reasons: in sports car you don't need space or luggage by definition. In this sense M3 is not a sports car either, since it has rather spacious rear seats and trunk.
Sedans are structurally stiffer than the Coupes, especially the ones w/o the folding rear seats. A lot of racer prefer the 4-doors over the Coupes (look at Turner winning cars) partially for those reasons.
M3 is a two-door because it was cheaper to design a one style model (since it is quite different from the regular E46) and they already have a premium 4-door M car, the M5. E36 had both, but then BMW didn't have an M5 available during that period.... Building a 4-door M3 would have cut into the sales of the E39 M5. But for the next gen, BMW is going to build M Sedans and Wagons alongside the Coupes/Cabrios.
Also, take a look at the EVO and STi, both are 4-door sedans but with a soul of true racecars.