Ruben's not the most devoted Team Rocket member, he sticks to the organization because it provides him with food, shelter, money and pokemon - basic needs.
Working for Team Rocket is certainly easier for him than coping with an actual boss, actual co-workers and all the unnessecairy paperwork that comes with living a proper life.

... I guess he'd still a bit annoyed if someone would tell him in his presence that Team Rocket is weak shit - he is a member afterall.

I guess Nate (his Houndoom) just thinks they're both incredibly childish.