rare candies have always been and shall always be one of the prized items of pokemon
however, with the introduction of EV's, rare candies have become looked down upon by serious trainers.
This is what they do and what we should know about them:
-raise a pokemon by 1 level INSTANTLY. activating any effects such as learning a move or evolving
-cannot be bought (at least with the BASIC amount of currency existing in all games)
-very small amount exist in one map and once they are gone, they're gone
with the introduction of EV's players became more inclined to actually raise their pokemon through battle as opposed to candies. This was, of course, do to the fact that rare candies did not supplement ev's for the pokemon, thus the more rare candies used the less EV's could be gotten.
what are EV's:
-stands for Effort Values
-everything a pokemon battles grants some amount of EV's
-a pokemon can have a TOTAL of 510 EV's
-every 4 EV's grant +1 to a stat
-one stat can have a maximum of 255 EV's
-every pokemon involved in a battle recieves the same amount of EV's period
for more info go here
http://bulbapedia.bulbagarden.net/wiki/EV
anyways, the reason rare candies can be considered 'bad' is because it takes away all those battles a pokemon would have to be in to acquire that level
This means,
the higher the level, the worse the penalty is for using rare candies
so in essence, rare candies AREN'T bad if used early on, but are more damaging to a pokemon stats if used in higher levels