title_body: What is the advantage of a stretched-X frame over a true-X or squished-X design for racing?
upvoted_answer: I'm not a frame designer, but one distinguishing characteristic comes to mind as to why stretched-X frames are more common.
  
  As you mention, a stretched-X frame has a longer front-to-back dimension than its left-to-right dimension. In addition to providing the rear propellers with cleaner airflow when in forward or backward motion, this results in the moment of inertia about the pitch axis being higher, which means that it requires more work for an external force (e.g. wind) to change the pitch of the quad inflight.
  
   (cit.)
  
  This also means that, as you noted, the pitch axis has greater control authority (due to the longer moment arm) which allows the quad a more quickly accelerated response to the pilot's commands. The differences in control authority accessible to the pilot can be smoothed over with rate profiles and PID tuning so that the quad maintains the stability it gets from the stretched-X design while responding uniformly to pitch and roll commands.