The only 2 reasons I would want Halo 5 to have MLG alongside with it are;
MLG in past Halo titles (H2, H3, H:Reach) have all had a strong competitiveness about them, Halo 3 more-so however. This ensured that the developers worked more on making the game a strong competitive game in some, but not all areas. With Halo 4, there is next to no proper competitiveness, no incentive to win at all, because at the end of the game the losers only get ~400exp less than the winners, and they'd still be getting over 2000exp. If Halo 5 were to have MLG, 343i would focus on making the multiplayer a lot more competitive in areas (probably separating the playlists like in Halo 3) which would keep a large portion of the Halo fanbase happy, while keeping the casual play-to-have-fun players around in their less competitive playlists such as Big Team Battle.
The major problem with Halo 4 is the lack of players still playing the game, this was almost entirely due to the multiplayer being an extremely casual one (in my opinion). Reach saw the beginning of the end, the credit system killed a lot of incentive to win, however it kept a lot of the players hooked because of the fun it had and the variety of new gametypes that it offers. That and the wish to become an Inheritor spartan. Halo 4's initial rank system was utterly awful, 343i didn't learn from Bungie's mistake of having a credit/exp based rank system, and only made it 130 ranks which were reached in no time at all. The half year delay for a 'proper' rank system which should've been implemented in the first place, the CSR, had too many flaws that the many players that came back for that just left because it wasn't what they were expecting from a game like Halo 3. Though I admit Halo series should move on and I don't want a clone copy of Halo 2/3, the ranking system was far superior and kept a competitive edge with players that made most people want to win no matter what. This isn't what Halo 4 had, though it should've. Getting to the point, by allowing MLG back into the arena with a proper rank system/separation of playlists, a large portion of Halo fans would come back to the game and would flood the servers for many many months, and possibly years. This being unlike Halo 4 which in a continual player stat is an utter failure.