Trav,

That is a very common tecnique among rock writers. Actually back in my band days that is how everything was written. the band would come in, jam away for a while, then later the lyrics were written by myself and usually one other of the members.
For me,my move to Nashville involved having to get better at it as that is the nature of this town. And while the over 25 writers with number one songs that I have written with were often drawn to me because of my R&B styles, the reality is that I always had to step up to the plate and get better as lyrics as well.
But we always try to work with others who compliment our strengths and weaknesses. If you don't feel as accomplished at lyrics, by all means always find others who might be able to take it to places you can't. 70% of writers are lyric only so you are in a good percentage when it comes to your skills.

Good luck and stay at it.

MAB