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You may call them horribly incompetent, and you may call Giovanni horribly dumb to keep them employed.
But their constant attempts to capture Pikachu have, Directly and Indirectly, contributed to the continued survival of the entire planet on more than one occasion.
Additionally, the constantly keep Ash Ketchum on his toes, and prevents him from ever taking just one or two days to lay back and think about what he is going to do next, preventing him from reaching Red levels of competence and dismantling Team Rocket over the course of a few days.
In short: The three fuck ups, whether Giovanni knows it or not, are his biggest asset.
And damn stylish too.