Always loved playing shooters on a competitive platform, and Destiny just seems like a fun game to at least give it a shot for it, especially if they plan adding another competitive game mode.
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In order: Probably not, probably not, and no. Crucible is far too laggy to ever make it to MLG, especially without the other two modes you mentioned. Besides, it's pretty much just a run around and (insert special weapon here) everyone to death game mode, with only a few exceptions.