As we all know, whenever Yamato and Kosaburou appear, they always give everyone a really hard time before eventually being carted off to jail.  They have thrown Satoshi-tachi and Musashi-tachi in jail. They made just about everyone, including Musashi and Kasumi, cry in Re-volts, etc.

    There is a reason for all of that.

    Yamato is not only Musashi's rival, but also her foil, meaning the exact opposite of the hot-tempered redhead.  While Musashi is, well, a looser, Yamato is one of Rocket-Dan's elite members.  Where as Musashi is not really evil, Yamato is a
very bad person. As Musashi plans to capture Pikachu with a plunger, Yamato is using ingenious, sinister plans.  I could go on, but you get the picture.  Also, since Yamato is Musashi's rival, Yamato will forever be more than a few steps ahead of Musashi, just like Shigeru is several steps ahead of Satoshi.

    *Note, it could turn out that Kosaburou is Kojirou's foil and rival, but there is not a whole lot of evidence to support it at the moment.

    Now, since it is established that Yamato is Musashi's rival, how can you have a rivalry when only one person is shown?
Yamato and Kosaburou are periodically shown to continue the rivalry, just as you see Shigeru every once in a while to give
Satoshi a hard time. Since Musashi is not the main character, however, her rival is only shown on rare occasions.

    So, Yamato and Kosaburou appear on an episode to give Musashi and Kojirou a hard time.  But, since this is a kid's show, the bad guys couldn't win even if they had all the powers of the World behind them.  This would send the wrong message to kids, showing them that crime does pay.

    Thus, you have competent, dangerous criminals aggravating the incompetent, sometimes friendly criminals, with the Law
prevailing every time.

    Makes you wonder what Yamato and Kosaburou are doing when they're not in an episode, since they are elite members, after all. ^^