I do think it's worth a little bit more to purchase the Halo map packs, because along with them, you also got some user-created Forge maps. Hell, just for people to submit their own maps, I'd gladly pay $10 to access a menu solely consisting of those.
However, with the amount of game modes Halo has already, it's almost pointless (unless you are a really hardcore Halo player) to purchase any map packs.