"work around"

Dual Casting is the entire point of the job, and almost all of its skills are based around that concept.

A RDM that's simply using their spells will outdps a large portion of the players, simply due to how powerful they actually are and just how easy they are to pull off. Red Mage potential dps isn't much higher or lower than any other job in particular barring mch and brd, the problem lies within how easy it is to achieve that damage in comparison to those other jobs.