I think that professionnal writers don't, unfortunately, do a better job at designing rpg rules than amateur ones. When it comes to that topic, what is important is feedback, pros may have access to a broader audience for this. But, unfortunately, fanboys seem to occupy all the space in those playtests, and we are flooded with awesome comments... So the quality is not what is expected.
It is also my opinion that there is an area where amateur outshine the professionnals by several level of magnitude : scenario writing. Pros concern themselves with constrains that make their adventure duller than dull : number of words, respect a certain layout, sending the text on time, etc. Average adventures such as Red Hand of Doom or Paizo adventure paths seem good only because the rest of the production is mostly mediocre (to say the least), and the rpgers got used to this level of mediocrity from TSR days.
The only way, in my opinion, to get better adventures, is to publish amateur work : the way Dungeon magazine did in the past.
The only place where pros are really important, and cannot be ridiculed by amateurs work is illustration. Amateurs simply cannot compete there.
But on design and adventure writing, pros are no better, and can even be a liability to the hobby (the terrible quality of adventures for D&D4 is the main reason for the edition war in my opinion).