Question: why rare candies are bad?
Answer: Feeding a Rare Candy to a Pokémon will cause it to gain one Level, up to the maximum of L100. ... Rare Candies provide no Stat Exp; Pokémon who are raised using this item will therefore tend to be weaker than those who are trained in battle. If you've used Rare Candies on some of your Pokémon, don't worry.