Question for all Harry Potter fans.?

I was always confused about this. Especially in the later movies when everybody seems to have skill. Why does anyone bother with these stupid weak spells in a fight. If I was there I would be spamming Avada Kadavra. I saw Ginny use Reducto and it was freaking awesome. I would be in love with those two spells and that is all you would be seeing me use in a fight and the fight would be over in a second cuz I would just obliterate you with one of those two spells. So why docent every body just use spells like that in the movies?