The point of any big engined car is part refinement/engine noise and part performance. The 335 differs from the M3 in that it's more about silky smooth acceleration rather than raw rip your head off M3 power.
Just because you can't do 200mph in a car doesn't mean it doesn't justify the bigger straight six engine. 0-60 is more important to most people btw than top speed.
What this really comes down to is car enthusiast versus everyone else. Drive one and you might feel different.