with the recent nerf putting julien dance's motion values in (presumably) their originally intended place, double saber now actually incentivises doing things other than spamming the most awkward-feeling PA half i've ever seen, which has piqued my interest in the weapon. deadly archer gives an interesting and satisfying burst dps option, which has a fairly short cooldown compared to similar skills on other classes (such as spread shot and tricky capacitor). however, it depends on the whirlwind buildup mechanic, which has another use.
double saber's dps values fall far behind other fighter weapons, as well as other melee weapons in general. according to this very helpful spreadsheet, julien dance is still the best non-counter damage option, even post-nerf. it has a normalised damage of 772 for the whole animation but takes 171 frames, almost 3 seconds, to execute; even with the more efficient skip art variant, it only achieves about 276 dps. compare this to, say, the twin daggers' 389 dps with circling buzzard or the knuckles' 367 with thousand blows. you get a similar story with the double saber's normal attacks, step counter, and normal counter, all of which fall behind comparable equivalents on other weapons.
all this is made up for by the dps granted by having an active whirlwind. upon activation, the double saber whirlwind deals 32 dps to nearby enemies, and after a certain number of hits this gets upgraded to 80 dps, where it remains unless it is consumed by a skill or unless no hits are scored for a certain period of time (at least, from my tentative observation).
the numbers on the spreadsheet regarding whirlwind are actually a little sketchy, and seem to accord a much larger dps boost to it; i'm not sure where that comes from. but it seems like a level 1 whirlwind should boost the dps of skip arts julien dance up from 276 to 308, and a level 2 whirlwind should then bring it up to 356—still a little behind other fighter weapons, not accounting for things like mobility, flexibility, and counter damage. a level 1 whirlwind is easy enough to maintain: even if you lose it, just counter any attack or activate a PA. however, maintaining a level 2 whirlwind for that juicy 48 dps boost (over 15%!) requires you to choose not to use deadly archer while you have it up--in other words, you have to choose between whirlwind level 2 and deadly archer.
obviously, burst damage has its own value, so there'll be special use cases for deadly archer even if whirlwind wins out in the general case. but the question still remains as to whether there's value in spamming deadly archer as often as possible.
if we choose to use deadly archer when it's available, we can check and see that it does 1736 damage over 180 frames, or 3 seconds. this means that your dps goes up to 579 for those three seconds. however, since you lose your level 2 whirlwind, your (ideal) dps subsequently drops to 308. presuming you spend no time countering attacks and no time recovering pp (or at least, presuming that those things balance out), we can simplify things and approximately say your dps remains at 308 for until you get deadly archer again, and immediately use it. since we're comparing this to a solid, steady dps of 356, we can say that by choosing to use deadly archer, you gain 3 seconds of 223 bonus dps, and then lose however-many seconds of 48 dps.
in order for your dps to break even, then, you need to have deadly archer up often enough to gain more overall dps than you "lose" by not having whirlwind level 2. to put it differently, deadly archer gives you a free 3 × 223 = 669 damage advantage over whirlwind 2, but then for each second you spend not using deadly archer, whirlwind 2 comes ahead by 48 dps, slowly eating up the difference. 669/48 = 13.94, so if you spend 14 or more seconds between deadly archers, you're losing dps.
conversely, if you're hitting a boss during a down phase (especially a break phase where damage is greatly magnified), you'd need to use deadly archer and then have 14 seconds still to spare afterwards for it to be a bad decision.
my main question here is what options actually let you accumulate whirlwind energy faster, since there's no clear meter for it and in my testing i've seen whirlwind come up both very quickly and very slowly. is it purely based on number of hits, making skip arts julien dance even more desirable as an option and giving some utility to unchained circle?