Most people just learn a song for the sake of learning a song or just simply because they like it. Well what if I was to tell you another reason for learning a song? How about learning a song for the purpose of learning a specific technique? I remember when I was first learning how to play guitar. I would see a guitarist from some famous group do a certain technique. I would sit up and say: "wow! I have got to learn how to do that!" So I'd pick up one of the band's albums and the sheet music for it, just so I could learn some of their music. I would learn a few of my favorite picks of their songs, usually the ones that had the techniques I wanted to incorporate into my own playing. Then of course something else would come along and I'd do the same.
I learned how to fingerpick fairly early on in my learning years. I started out with a song that used only a couple of fingers (thumb and index) and then I would learn a song that utilized three fingers, and so on. Then I saw Jimmy Page play with a pick and his fingers. Then I obviously had to learn how to do that. I've learned bits and pieces of numerous songs and it was usually for the purpose of learning how to do a specific technique. A lot of the songs I never ended up doing with any of the bands I was in. But the techniques were applied to some of our originals and even applied to some of the covers we did by other artists.
Most people think that the process of learning a piece of music for the sake of learning a technique is just a learning process that is usually applied by beginners. But even experienced guitarists can benefit from this approach, especially since there is always some innovation popping up in the guitar world. Like when Eddie Van Halen came out and did "Eruption." All of a sudden everyone wanted to do hammer-ons. When Randy Rhoads came out, everybody wanted to blend Classical guitar with hard rock. When Yngwie Malmsteen came out, everybody wanted to do lightning fast arpeggios. I'm not saying that someone should learn the trick of the day just to keep up with the trends but they should at least be willing to continue to improve themselves as musicians. To be a well-rounded player, one should learn multiple techniques. And this could be done one song at a time. So every song could potentially be an exercise.