Is it worth playing Ground Zeroes before Phantom Pain?
Is it a glorified demo or does it add anything (content, inventory or story-wise) to the Phantom Pain?
'Zeroes' does add extra items to 'Phantom Pain' and some character's for your base, but none of it is essential.
Cheers
While a bit demo:y, I think it's worth it. Lots of tapes to listen to (bring you up to speed on Peace Walker) and there's a good amount of gameplay. Really easy to find new ways to infiltrate the base in the main op, as well as play the extra levels in different ways.
Even if you skip it, it's story is summarized quite well in The Phantom Pain.