>>12379796It's been explained many ways.
One way, is with out Pikachu, HIS Pikachu, they won't be full fledged members of team rocket.
Another way is, they don't want to be out of the limelight. THEY wanted to take over the show at one point and rewrite episodes.
Another is they actually care about Ash and his friends alot. When they help them out, they usually are more powerful. Remember when Pikachu and Meowth got stuck together and landed on an island of giant angry pokemon? Jesse's Arbok and Ash's Snorlax cleaned house and beat the fuck out of a giant Pidgeot and Rhydon.