The affect doesn't specify Active Pokemon only, so you can keep it on the bench and it ignores half the energy removal Abilities/Trainers.
The effect applies only when the card is being discarded from the active.
"If this card is discarded by an effect of an attack of the [R] Pokemon this card is attached to, reattach this card to that Pokemon from your discard pile."
Meaning if I were to use Camerupt EX's Explosive jet attack but had Burning Energy attached to one of my benched Fire Pokemon and used it as cost for damage, Burning Energy would not reattach to any f my Pokemon. If Burning Energy was attached to my active Camerupt EX, then its effects would kick off and be reattached to Camerupt EX.