Weav-vile, you brought up some interesting points, but the answer should be obvious. Parawhatchu or whatever it's called, because it's honestly one of my least favorite Pokemon from D/P so I honestly cannot be bothered remembering it's name, is basically the 'Pikachu' of the generation, being all cutesy and girly and almost... well, killable. Wouldn't you love to see Probopass just smash through a wall, come out with some witty, Marx-esque one liner and then gouge the little rat with it's nose?
Anyway, my point is, they're using Parawhatchu a hell of a lot more because it's 'cuter' and it 'appeals' more. Also because it has electric attacks, it can do a lot more than a basic Normal Pokemon could. Note that apart from Aipom, the characters rarely catch normal Pokemon, and when they do, they're either not used much or start using other elemental based attacks. (Eevee learned Shadow Ball didn't it? >_>)
Personally I find Buneary's crush a hell of a lot more interesting than that thing. :/
*still waiting on Probopass*
Speaking of the episode itself, I just knew he wouldn't be wearing his scientist coat. The writers like I stated previously were KICKING themselves for dumping Gary so he could be a wimpy Scientist. So they slowly brought him back in, now as a trainer- if they haven't given him a purpose in this series I'll lol, as it'll prove just as worse a plot point as the G/S Ball and Kurt.