Car maker like BMW do not put a SMG transmission into the M3 because it is just a toy (with the exception of the 3 series SMG2), the main reason is that the I6 engine require a manual gear box in the first place, the automatic will not do when it come to high-rev output engine with less torque at lower RPM. They have to lower the hp figure and increase the torque in order to fit an auto-box. That's why US-spec E36 M3 only come with 240hp because they have to impletment an autobox for US customers. The E36 Euro spec and Evo do not have automatic.

Similarly the M5 do not offer a automatic, in marketing stand point, the SMG is a "replacement of automatic" rather than a gimicky racing transmission. Imagine how many more M5 BMW could sell if they have the SMG avalible. Most people don't know how to drive a manual in America let alone heel and toe, match-rev downshift ... The SMG offer all of that to people who never touched a manual gearbox before.

In my point of view, after owning and driven a SMG2 M3 for couple of month. I can honestly do better than the SMG2 in shifting time if i really focus, rest of the time, the SMG will do better.