The show is really bad except for the few episodes that actual comic book writers are in charge of (like the mid-season finale). Diggles and Laurel have gone beyond useless in this show and have absolutely no role.
I laughed out loud during one of his typical worthless scenes.
"You know, Roy, Slade, some guys I served with, some people are just broken. *hands Ollie his ringing cell phone that was 2 feet in front of him anyways*"
You can count on the writers to have him say something generic about his time as a Marine and then do something pointless just for the sake of giving him screen time because.. he exists. They looked like they were setting him up as an actual partner who could fight along side him sometimes back in Season 1 due to his experience/training.. then he just turned out to have no role at all. And don't get me started on Laurel.. jeez.