Just a note on this- few developers seem opposed to having to design around addons. Most see addons as a way to tweak and adjust encounters around the aspects players find most important.
Not too long ago there was a
Scott Hartsman AMA where he talked about what they learned from addons in making encounters tighter and more focused. Tankspot also had an interview with one of the WOW devs remarking similarly. Early 3rd party tools in games like EQ were also major steps forward in creating tight encounters for 989 as well.
Just saying, we haven't seen addons negatively impact games to any equal or greater degree than the benefits brought in player options and developer crowd sourcing.
Kind of a
myth, bro.