well that perhaps is right........it can also help you to jam with other musicians when you know which scale you are playing in.......especially free jazz stuff is very tricky that way.
But IMO you shouldn't stick to hard to the scales.......and I know lots of musicians who have so much knowledge of scales but just write boring predictable music because they aren't brave enough to leave the scales and just play what they think could fit.
so....scales are important basics which can help you play.........but imo it's not necessary (sooner or later you'll find scales youreself while jamming)
as keith said: scales don't limit you if you know HOW to use them.......but if you don't know....
but when I hear you say: "And also I would like to know the common scales for writing metal songs etc..." it just screams for limitation!
It sounds like........hey lets write a metal song, so I have to take the metal song scale with the tones I am allowed to use.......
uke: