I'd imagine that TR does help Ash gain experience, but it hasn't been stated.
It makes a lot of sense that Ash gains a lot of experience from Team Rocket. If you think about it, TR appeared more and more up to the DP series. Ash's skills continued to peak as the series went on. Many would argue that he was a fairly experience Trainer during DP, which was when TR appeared in about every episode. During BW, when Ash seemed to lose some of his experience, TR also appeared considerably less often than they did in previous series.
There's certainly a correlation. I don't think it's intentional, though. TR isn't really the best punching bag for Ash and his Pokémon because they are so easily defeated, and their main purpose is to serve as antagonists, usually for filler plot.