Pachirisu helps your Dialga and Palkia matchups. It resists Dialga and slows down their deafen lock enough to give you time to set up, and it hits Palkia for weakness. Mesprit helps your Gigas matchup, denying them a sacrifice for a turn (or more if you can hit your SSU flips) which can cost them their Gigas mid-game. Crobat is like a bench-hitting searchable pluspower, great for 2-hitting 50hp pokemon on the bench (like Baltoy). Skuntank is okay, but I wouldn't waste the space with the way the format is pretty much dominated by Galactics right now.
Oh and some people tech a 1-0-1 Machamp for the Palkia and Dialga matchups, but you end up having to run Multi and Fighting energy which basically makes Aqua Stream useless.