I don't have data to back this up, but I've heard from e-sport pros that with your RTS and MOBA (sc2, lol, dota the biggest right now) the mass majority of audiences for tourneys and streams are by people who don't actually play, but are familiar with the genre.
While FPS e-sports like CS:GO, CoD, Halo etc, which up until recently with the surge from MOBA games, have always been the dominant e-sports as far as viewers, have higher %'s of audiences who play those games.
To me, the difference between a "competive" game and an "e-sport" is that although both are high skill, clear ranking system and in-depth balanced games, but an e-sport also is entertaining to watch for audiences (both those who play the game and those who don't) Audiences provide money, players get sponsored and you have "proffesional" gamers.
I would argue that a game like Halo or CoD fails at being a good e-sport because they are terrible to watch and follow because the nature of the game. It is very hard to follow what is happening in these games.
League and Starcraft however, are very entertaining to watch because they are very easy to follow what is happening. Yet, you can't just label FPS games as bad entertnment for streaming, because is also a very popular and despite very fast paced and hectic action, remains easy to follow. CS was clearly developed with intention of being an e-sport. And unlike other FPS's, CS keeps "dull" colors and small maps, which I think is necessary for creating a viewable/entertaining show for audiences.
Assuming M:BG is "competitive," which I think most of us agree it could be. And assuming it is big enough to maintain a large playerbase, I think it could easily become an e-sport. This genre is obviously not Lol or Sc2, but if you compare M&B combat to CS, you have A) dull colors, easy to switch from player to player without bleeding eyes B) small maps that will keep all the action and potential strategy simple. But also C) a combat system where the point of interest is always players fighting point blank against each other and next to their team mates (which is how LoL and Sc2 plays out as well, most action can be viewed from a single angle.