Hello fellow Dancers!
I've been playing with Saber Dance for many moons, and have had 5/5 Saber Dance for much of that time. It turns out . . . not so useful!
Saber Dance's time decay is much more rapid than the original report on ffxiclopedia. After parsing against a Hpmede in sea (and some kparser adjustments), I've pretty much convinced myself that it works like this:
When you activate Saber Dance, you have a 50% Double Attack rate that decays to 20% at about 1%/second (so 30 seconds.) It then remains at the 20% rate for the rest of the JA's duration.
Graph
Control test with 5 minute usage.
If you do the math, this means that 1/5 Saber Dance grants 21.5% Double Attack, while 5/5 Saber Dance grants 22.5% Double Attack average (and you activate it more often, so you suffer more JA delay). As far as playstyle, this means that you should try to stack Saber Dance with WS usage, instead of being sloppy about putting other JAs between it and a WS like I used to.
Edit: Oh yes, and corollary-
Step Accuracy is pretty much worthless. Steps get an innate +10 Accuracy over your normal swings. If you TP with near-capped hit rate (which should always be the case) then you don't need to swap any accuracy gear in for Steps at all.
Additionally, it looks like all the easily accessible Step Acc gear in the game (Hands/Feet/Earring) amount to about +20 Step Accuracy (+10% Hit rate) when combined.
Edit2: Oh yeah, and it looks like we may have gotten Accuracy Bonus IV. I need to test this more thoroughly though.