Onto the DNC issue, they spend large amounts of TIME to acquire that TP for reverse flourish. Steps force a lag of 60~180 delay units before it allows your round to continue and for little actual benefit because the monster is gonna die too quickly. Those steps prevents swing time, thus lower DoT. They can then choose to use their TP for DE's or spending large amounts healing. Similar to how a BLU can choose to cast Diss or Magic Fruit, but never both for the same MP. RDM's support power comes from our MP, our offensive power comes from our weapons and the 12MP enspell. Blowing 88MP on a Cure IV will only cause me to miss one swing, it will not subtract from my WS or other damage pool. And I can just refill it once per 9m.
Again everything has opportunity costs, and I mean everything. Nothing is free but some things are cheaper then others. For RDM we get the most while sacrificing the least. Everyone else, DDs casters and hybrids alike, must sacrifice something significant in order to do something else. And for some jobs the sacrifice will never be worth it (WAR/WHM and other silliness).
JAs have a delay, sure, but so does casting. Why minimize the impact of casting and focus on JA delay?
Steps are still a net TP gain. Spend 20TP for 4 Finishing Moves, RF for 46-61TP (depending on merits) back. I doubt you'd get the extra, say, 40TP in that couple seconds using the steps delayed you.
Also, why focus on Dia like it's the end-all benefit and dismiss the benefit of Steps?
DNC is not a DD, same as RDM is not a DD. That something they do for support purposes might lose an attack round or two in a fight isn't critical, especially since they gain more TP in the long run from it + enfeeble the mob in the process.
There are always tradeoffs, sure. By focusing on healing, a DNC isn't going all-out damage. That said, the process that they'd go all-out damage on would be the same. Using Steps = Finishing Moves. If you want big WS on DNC, you need Finishing Moves for Building Flourish. Maybe even Wild Fourish. So you still need to do Steps, unless you only want Finishing Moves every three minutes from No Foot Rise.
Delay or not, Steps are an integral part of DNC, offensive or defensive.
Same with gaining TP. You're pushing to get TP whether you're using it to cure or to WS.
DNC isn't perfect, but like I said, they're complementary for a backline healer. A frontline RDM is offering practically nothing (in terms of support) that a backline mage couldn't technically already be doing.