Duo still hasn't decided if Adachi is actually something of an idiot or simply very good at playing the fool, which means he's dangerous, because he's like Duo. He definitely treats Adachi the way he would think of himself, for that reason; keep at arm's length, contact only when necessary, but keep an ear to the ground and find out what he's doing.

He's a valuable contact and ally because of his connection to the Phantom Thieves, who have themselves proven to be a dangerous and powerful figure in this world already, and he would much rather get close to them as their ally than find them a distant enemy. He wants to believe in their will for good change; he just has to confirm that that's what they are, that they aren't some misguided revenge seekers or anything like that.