On a slightly different note, considering the original poster most likely missed the fact that Royal Road have three effects and not one...

Many debuffs in this game can't even be healed in the first place, so using an Esuna on them is wasting time and mana. If there is a small, white bar over a debuff then it can be healed, if there is no such bar, then it cannot be healed. This is important, since the same buff (like poison) can be healed when it's used by one mob, while it's incurable when it's used by a different mob (mostly bosses).
Others are so short that due to ping, cast time on Esuna and global cooldown, they are almost sure to end before you'll actually cast the spell (stun, silence and pacifism usually fit this bill).
Then there are mob fights where using Esuna is just a sign of an inexperienced healer. For example, curing a typical poison in a fight against three or four mobs is just going to waste that Esuna, as the mob will poison the tank again in the next few seconds. With the damage that poison would inflict even if it did run its full course, it's better to just spend the time and MP on casting Esuna go towards either HP healing or damage.