You know, back in the Johto seasons, that really WAS the reason they were kept around. In one episode, they meet a Team Rocket recruiter who tries to get them to join, only for them to find out they've already been let go after a mountain of unpaid membership dues piled up. In the end they just end up being rehired since there are so few recruits (gee, with the trio being the interregional face of the organization, who could guess why), provided they work off their debt on zero pay.

How do they keep their job? They wasted at least billions of pokedollars on one kid and his pikachu.