I don't disapprove of Rare Candy training, I just find it to be pointless.
Long-term (lv. 50 and above), EV's are vital. If the Pokemon hasn't actually been fighting any opponents, and thus hasn't gained Stats bonuses through EV's, it will be significantly inferior to a Pokemon that has reached Lv. 50+ through battle.
In this context of player vs player battles, the person who uses rare candy training is only cheating themselves. Yeah sure, your'e winning now, but only through level superiority. Once your opponent reaches the same level as you, you will find yourself outmatched.
this guy clearly just wants to one-up other players by over-leveling his opponents. Don't waste you time on him, unless you intend to train your pokémon to his level and beat the crap out of his inferior pokémon.
As soon as i learned of EV's, I immediately decided to only use rare candies to level up and evolve bug Pokemon (low-level evolutions. Complete pokédex!XD!) , then sell the rest (5000).