MOBAs tend to be focus on gameplay mechanics with three to five players per team. The concept is that a handful of players will need to balance between the various attack paths, and a limited amount of cooperation is possible before you start leaving zones uncovered. It also means that one problematic player can tank an entire team.
This will not change in the official DOTA 2 game, but Valve is expanding the limit for custom games. At The International 5, Valve announced that those games can support up to 24 players. The first public game was a 10 vs 10 match at the end of the fourth day of the tournament. While I don't play DOTA 2, it sounds like Custom Games in DOTA 2 Reborn are a lot like StarCraft Arcade, where users can create mods like dungeon crawlers and even objective-based games. In this case, an increased player limit would be very useful. I am not sure whether it works for the base game, though — maybe it works better?
This patch launches next week.