Most people use repels and avoid Trainer battles because usually they are in a hurry. Getting into a battle might distract them. People want to just get to their destination as soon as they can so they avoid battles.
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Repels make WEAK not strong Pokemon not appear
Yes you can
I don't think. I think if you use a repel only legendary Pokemon will appear.....I pretty sure. you might have to look it up if my answer is not right.
It repels Pokémon in the wild, up to the level of the highest Pokémon in your party.
You would need to hack the game, but you can get shinys much easier w/ the pokeradar and max repels in diamond and pearl