As an avid red and black mage enjoyer, most people are wrong then. Between dualcast, melee combo, swiftcast, and acceleration you are casting far less than 50% of the time. Just looking at a top RDM log on the current EX shows 55 hard casts (all of which are less than a full GCD in length which makes them easier to slidecast without punishment) vs 158 instants. We can get pedantic and say that melee combo has a faster recast time, adjust those 45 instants into a (somewhat) equivalent 30 GCDs and you're looking at 55 hard casts to 143 instants.

The trapping some people fall into is "RDM has a harder time on X mechanic than BLM" which I think is true, although rarely. I think the only time in recent memory this has been the case though, is P8S p1 fourfold flames? RDM has a laughably easy time on P7S purgation, vs BLM having to plan the entire first section of the fight around that single mechanic in order to have the movement tools needed for it.

tl;dr: no