I don't buy the "I play for fun" argument. We all play for fun. It's unhealthy to think of fun being exclusive to "close" matches.
That's just like, your opinion, man. Some people want to focus and learn and get better. Some people just want to farm some creeps and gank some n00bz. The problem with smurfs and such is that they force themselves upon those who might not want that at all, cheating the system in the process. Like, if I actually seriously cared about improving and being 'good' at DotA, I probably wouldn't be at 2-3-4-5k mmr in the first place, don't you think?