A common thing to do amongst guitar and bass students is to just learn just one part of a song: Usually the catchiest section, that they could play to someone who would recognise it and say “Yeah that’s…..”
Well, if you’re ever going to want to play guitar/bass in a band, any band, you’re going to need to be used to playing songs all the way through.
And what if that band was Green Day?! Here’s the most incredible, exciting reason to be a guitarist who is always prepared! – http://www.youtube.com/watch?v=1hVt05VXs3o