A) You can play a game at your actual skill level, which requires your team to communicate effectively and play together. This requires someone to take the lead and coordinate the other players. That just doesn't happen in drop in games. It is incredibly not-fun and frustrating to be playing decently yourself, but be anchored by incompetent team members. You get guys who want to play their own way, or just aren't interested in "taking orders". Plenty of arrogant dudes that won't listen to someone that is a lower rank.
B) Play at a level where you can essentially carry your entire team to victory because your skill level is so much higher than the enemy. You get to fuck up a bunch of newbs and win the game, while your team mates are drooling at spawn.
B is definitely more fun. Especially when the people smurfing can get their fix of A in an organised match with an organised team, outside of match making.