Out of all the members that have ever been in Metallica, Dave Mustaine is by far the most interesting and controversial character. Yes, he comes across as a self-serving asshole, but whenever he spoke, it was way more intriguing and pointed than anything Lars, Kirk or James ever had to say. Not to mention the music and lyrics he writes, which are full of the snark and satire that tweaked your sensibilities, qualities that just aren't there in Metallica's music.
And therein lies the real reason I believe that Mustaine was booted from Metallica... that fantastically outsized ego. Of course this is all speculation, but I got the sense that Lars and James didn't want such a strong personality in their midst, and ended up using the alcohol abuse as a facile excuse to cut him loose. As good a guitarist/songwriter as Mustaine might have been, he was kind of a threat, whereas Kirk Hammett slotted much more easily into the vision that Lars and James had for the band.