Rivals! Variety!

A rival in Pokémon training is a good thing to have. By comparing your trained Pokémon and battling style to theirs, you can often pinpoint flaws in your team and devise new strategies. All it takes is a healthy thirst for competition and desire to improve yourself.

When you have rivals that you battle regularly, you may find yourself hard-pressed to surprise them. Having a team of regulars is fine, but a variety of Pokémon that you can use in your team always gives you an advantage over regular opponents.

Some of the trainers I've battled with have been very faithful to a regular party of six Pokémon, and my variety of trained Pokémon almost always overpowers them. Especially during rematches.

Think of it like this... if your opponent knows what is coming, they can plan ahead for your team. Battling you may become like preparing for an in-game Gym Leader or Elite Four Trainer. Don't make it so easy for them!

I always try to break what my rivals use regularly, so they're forced to rethink even their most basic strategies.