I would find it helpful to include lyrics, so its all in one place synchronized with notation and it helps me in learning some songs. They are fairly easy to find elsewhere, a lot of band web sites provide their lyrics, but not with notation. I'm sure you know its quite easy singing along to a song, until you try and play it and sing without the lyrics in front of you karaoke style. As with the guitar parts if you don't have the lyrics and their timing well memorized it wont flow together and you wont be able to embellish your own style into the song. So for me lyrics are very important to learn along with guitar parts.

As a small critic, I don't really like them included in backing or jam along tracks, as I like to do them myself or with a singer once at that point.