It's a pity to have Growlithe fight for evil purposes.. moreover, if Growlithe is on Team Rocket's side, it'll be easy to catch Pikachu, and the editors probably have to think of creative ways to beat Team Rocket for ALL the episodes Team Rocket appeared in..
if TR possessed weaker pokemon (like now), the editors don't need to think of better ways for Ash to defeat TR except to have Pikachu blast TR off with some electric attacks.. I think editors just didnt' want to stress their minds out o.o
well, that's just my opinion though, not necessarily true