The lights are on
Peace Walker is a spin-off, first of all. Second, it does fill in a gap in that you play as Big Boss in the founding of Outer Heaven. The story is great, and the game brings a new gameplay mechanic where you get to manage an offshore base called Mother Base, which is Big Boss's base before Outer Heaven. As far as contributing to the series, it really just fills in a gap in the series' long history. I think it's worth getting.
By the way, I'm the biggest MGS fan, not you.