1) It's not fair for the other members, esp. in duels
2) It's strictly against the rules to use it because it's cheating
3) It's cheating because you can easily win with it
4) It's more of a dark charm and we're not trying to be like lord voldemort
5) There is no way to get rid of it once it's on you
6) It's unforgivable; there's no forgiveness on those charms.
7) We're not trying to kill each other, it's a friendly duel
8) Immediately get in detention from using those curses
9) It shows that you just want to win and not to have any fun
10) It shows disrespect and is a pathetic thing to use in a match
11) It's cheap way to win a match
12) It hurts people around you
13) It shows that you are not honorable and your are just looking for a quick win
14) You can only cast spells you know, unforgivable charms are not taught
15) It's a disgrace to ravenclaw