I don't really get it either, no one defending Halo has made a valid argument. Halo takes skill and talent, and practice to get good at, but so does every other shooter. I am a huge fan of Halo CE, one of my favorite games of all time, and a great 2v2 Competitive shooter to boot. But, it's still essentially just a slow to medium paced Quake with less map and movement complexity, and easier aiming. Great for consoles, but no where near the stratosphere of competitive merit of Quake 3 or Tribes.
Reading things like, map knowledge, team coordination, or knowing weapon drop times as an argument for Halo's competitive merit is a joke when I see someone land an amazing Disc shot at a pixel 100's of yards away, while skiing down a hill in Tribes, and they do that while doing all and more of the "unparalleled teamwork" that the Halo series has.
I think Halo 2 ruined Halo, the way people think the Halo and CoD franchises ruined FPS overall, so the hole goes even deeper. What I consider to be the most skill based and competitive console shooter ever, Shadowrun, got played by hardly anyone and then was buried under Halo 3 and then CoD4. Ever since then, its been the same shitty CoD games and its clones and whatever Halo has been trying to do since. Even the shitty original Xbox ports of PC shooters like Counter-Strike, RTCW, and Unreal Championship, have more competitive merit than the current Halo and CoD titles.