Scales are still confusing to me.
When I sing I don't know anything about scales. Nobody suggests me to sing in mixolydian, phrygian or anything like that. But, when I play guitar, it is very different. There is no little chance of someone suggesting to use a certain scale. I try to improvise without much knowledge of scales. I would say I don't do it perfectly but, I have some success.... My fingers feel comfortable following the music and "improvising". More or less the same as I try to do while singing.
What is so different between improvising using your voice or using a guitar?
One impression on my side is that you need mostly ONE scale which is transposed to any base note (CDEFGAB), and there will be a little percentage of songs that will require you to memorize "something special". That ... and blues. For example, and I may be very wrong, for a typical "Flamenco" progression — as Am, G, F, E — it feels to me that I am free to use a "normal" scale for most parts, except for the E chord on which I can memorize 4 or 5 nice patterns.
On the other hand, I know several friends with a lot of knowledge of scales. They are capable of describing the intervals: "colors", base notes, support notes, fretboard patterns, fifths, thirds, sixths, and a lot of elements that I don't know very well. But... when they try to improvise, I would say, they don't do anything great. Too much blah, blah, blah, but the lack of feeling, taste, sense is evident. It seems like they only move the fingers through the available "tones" from up to down and back again.
Is it really helpful to know scales, or is just some practicing exercise to learn more about theory?
EDIT: Thank you very much. A lot of great answers. I didn't want to suggest that scales are useless. I just wanted to have an answer for a long time doubt: "if scales are so important, why singers don't care about it". Yes, I consider theory is important and one great thing is to have a common language with other musicians.
But... as a singer develops a natural connection with its own vocal chords, an instrument player can develop a natural connection with its instrument board and be less worried about theory. I think this have happened since the beggining of music. At same time, an instrument player that have all the knowledge about his instrument can be still a newbie compared with the naturally given. Of course, I can still be wrong.
For now, I will continue offering the guitar to the next theory guy. Someday, I wish, I will find the one that express this theory with his heart and is capable of making it sound.
BTW, I'm not a bad player, if you play me your chords, I can guess a fitting scale for it. If you also hum a little of the melody, I can see the scale along the complete fretboard... and I thought that is normal for every player who have been practicing enough time. Probably thats the origin of my confusion