I think it's a financial issue, and probably a good thing. People are thinking about this in terms of association, and while I see that point, it also occurs to me that the "X-games" are a much bigger label, have been around longer, and have deeper pockets. This alliance gives E-sports access to more resources, and a wider audience, not to mention deeper pockets for putting on events and competitions.
I'd imagine that the two are going to split up again once E-sports gets a bit bigger if this works, but right now your pretty much seeing the X-games giving "Major League Gaming" a boostie so to speak.
For the record, while many people disagree with this, I do not consider video gaming a serious sport, at least as it stands now. That said I do believe it can be a serious competitive pastime. I personally put it more in the league of chess, poker tournaments, and other competitive games. Yes a group of people can put in some huge amounts of time playing games to get ready for a tournament, but the same applies to other things less chess and other similar games with competitive aspects. It's just that one type of game is played with physical media, and the other is played electronically.
The big problem I see with E-sports, is the same one that applies to other forms of competitive gaming, and that is that it's not very exciting from the spectators perspective. Yes there are some people who can enjoy watching other people play StarCraft for example, but it's not something that rivets tons of people to their seats and is going to have people so psyched up that they say decided to start "fantasy StarCraft leagues" where they pretend to recruit the best players to put together fantasy teams and such for example. It's sort of like poker and chess, it has a niche, and it has been televised (as had pro-gaming occasionally) but it's small potatoes compared to sports.
One of the big things I think that hold back pro-gaming as well, and will continue to be a problem (aside from the rest of this discussion) is the human factor, or relative lack thereof. As a general rule when it comes to *most* games (but not all) the computer is pretty much king. The only reason a person can say beat a computer at a game like "Street Fighter" is because even on the highest difficulties the game is programmed to be beatable. Barring exploiting some kind of glitch, if you just flat out unleashed the full power of a computer on real people for a lot of these games (there are exceptions) it would be literally impossible for someone to win since no person can react faster than a computer can when it comes to this kind of thing. In comparison something like Chess is "human" enough where it wasn't just about the technical aspects of the game, and it took a long time to come up with a computer that could actually reliably beat human grandmasters and such. In comparison if a company like Capcom just flat out designed a fighting game without creating the AI to be beatable and win at any costs, even guys like Daigo, Justin Wong, or whomever the best are probably couldn't even touch it unless they found some kind of glitch or hole in the AI to exploit (beating it on a technicality rather than outplaying it). AI tends to be stupid in a lot of games like "StarCraft" and such as well because it's designed to be "challenging" not to just flat out overrun the player as a computer is probably capable
of doing, if Blizzard for example just flat out set out to make StarCraft unwinnable for humans, I'd imagine they could probably do it. This point actually adds to the entire mystique of the entire thing, the nature of chess for a long
time was something that couldn't be automated... and even now there is still a lot of question as to how often even a supercomputer can beat an actual grandmaster (most demands for a rematch have been rejected, largely because the point was more that the computer could technically do it, not how often it could), and even so computers were still programmed with algorithims based off of people and the analysis of countless games. It wasn't a matter of say the computer being able to say set build queues down to a billionth of a second, or simply react faster than a human hand could possibly move on a controller. I suspect for this reason it will be a very long time before anything electronic that is reflex, timing, or real time based will catch on in the mainstream even if it could be made flashy enough to be fun for the average person to observe. The very fact that there can be an E-gaming controversy over "botting" (ie having the computer do the work faster and more accurately than a person) sort of demonstrates the problem. People want to see things that it requires people to do.