1: Many will fuel illegal trade of accounts, contributing to a black market where account trading are vivid.
2: Reducing the effect of the matchmaker, as their real skill level are much higher than their account think it is.
3: They will not give their best in their ranked games, as they are only "training".
4: Either dominate or feed, they will contribute heavily to games being less fun by either being way too good or way too bad.
5: They have a free card of toxic behaviour, as a permaban will have no effect whatsoever.
6: They give up easily when they dont see any point of playing no more, that LP arent important to them anyway.
7: They can troll around in games wihtout any consequence for their main character.
8: They contribute to to the culture where Silver and Gold players arent worth anything, and can be used for smurfs own entertainment.
9: They reduce the impact of players in Silver and Gold, ruining the fun of the game.
10: Smurf accounts falsley boost the player numbers of LoL, making it harder to tune Qs correctly.
Now, let the smurf-mob downvote and start flaming!