Its basically mandatory later on if you are going to do enough dmg to easily kill a monster, and if you can do it well then ur not a good meleer...so practice.

BLMs and WHMs(and RDMs too ) can cast a spell of the appropiate Renkei element right after the effect of the Renkei shows, this will be called a Magic Burst, this will even add more effect and dmg, at later lvls, if done right it can take out 3/4+ portion of the monster's HP. So its mandatory.