Yea Bottom has 2 people because of the Dragon. Say you gank someone bottom and the jungler is there. Having 2 people+Jungler is pretty much a sure dragon. Now if you had 1 person down there and they had 2 bottom and you kill one, you can't really act on it and take dragon because you don't know where their jungler is and it would still be 2v2 and you would be taking dragon aggro.
Dragon is actually worth more then a turret kill on team gold so it matters quite a bit. 190g for each person on your team is pretty huge it's like 9 minions-ish or 45 minions because it's x5 also the person who kills the dragon gets 25 so it's really like a 46 minion swing.
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Whatever...