A competitive game doesn't really have to appeal to the top %. They'll find their own strategies to give them the edge even if it wasn't specifically designed that way in most cases. The base gameplay only needs to be robust and things will naturally fall into place, even if competitive play only has a few viable strategies. As long as the game is popular and fun to watch, I think it has a chance. I think that something like melee: battlegrounds can appeal to a casual audience. All you need to do is left click and hope to cleave some fools in two.
I must say that I vastly overestimated how "fun" warband is to watch. Whenever I tried to show people, they were just overwhelmed with everything happening on the screen. I think that's a hurdle that M:BG probably should address, since it probably won't offer the scale/"awe" of other medieval-themed games of developers with larger budgets, regardless of its final quality.
There isn't actually any correlation between successful e-sports games and well balanced games. I've seen well balanced games fail, and unbalanced games succeed. The thing to remember, is that not every e-sports scene is the same as the next, they're often comprised of players with completely different expectations. You've got games like Super Smash Bros(no offense San), League of Legends and Call of Duty at the same events as Street Fighter, Dota, and Counter-Strike. The depth and balance of these two categories is completely different, yet each game is quite successful. Hell, even WoW, which has some of the worst balance you can find, has(or had) an e-sports scene.
I agree with most other than SF being deeper than smash of course (both aren't *that* deep, or at least SF4). I was a casual player for smash from 2002-2006, was referred to a venue ~2005 (since it's never really fun to beat friends who don't own the game) that I went to occasionally, but didn't really start actively going to until 2008 when the next game came out. One thing usually leads to another most of the time, since most start out playing casually until they find worth in improving even more and playing with those of similar skill level.