Problem is, when at a party and I pick up a guitar, I bust out a riff, everyone gathers around ready to listen, and at that point I stop and say "that's all I know of that tune" then start another riff.....rinse and repeat.
And yes, I know that there are many songs that are hard, and you really SHOULDN'T try to learn it perfectly (you can play the CD if you want to hear it perfect), but rather add your own personallity to it.
That's true to a point, but there are valuable lessons you can learn by figuring out EXACTLY what's being played on a record all the way through.
As guitarists we tend to focus too much on solos, but understanding the way a song builds and changes all the way through is far more interesting to me and frankly is the reason I get paid to come back time and again.
Here's a great example of how pros arrange a simple song, where a hobbyist might just play the same thing all the way through thinking they've nailed it;
Gibson Riff Building Session - YouTube!