like all bonuses, DA on WS is best expressed as a % bonus if you're trying to compare X% DA to Y something-else. however, you have to look at the WS to see what % WS DoT you'll get from DA.
abstracting away from everyone's favor topic of 2008/2009 (the diminishing returns of DA)...
like you said, you can only DA on the first 2 hits of a WS. DAs are like additional hits in multihit WSs, in that
a) they have fTP
= 1. fTP is what TP affects in some WS; for example, SAM WS have a large fTP modifier, which is what makes hagun good. WS gorgets also affect fTP, by adding .1 to it. fTP only ever applies to the first hit of a WS (edit: which is another way of saying that DAs and other additional hits in WSs have fTP=1. fTP is a multiplier; it not applying is the same as it being 1, since X times 1 is X).
b) they are affected by so-called secondary mods, or WSC
so, for example, if you double attack on tachi: gekko, the 75% STR mod does affect the additional hit, but hagun/TP do not. in order to see what % bonus you'll get from this, for a 1-hit WS you can multiply the first hit by fTP and then divide it into 1 (the 1 extra hit). so for tachi: gekko w/ 200%TP (100% TP with hagun has the fTP of 200%TP), it's 1 divided by 1.875 equals 53.3%. so, 5% DA would be 2.6%~ WS DoT
...but it's a bit more complicated, because the first hit of many WSs gets both an accuracy bonus (true of all WS, and basically autocaps the ACC for that hit) and a pDIF
(what ATT does) bonus. so taking that same tachi: gekko example, we get that 2.6%~ WS DoT only if ATT is capped relative to your target, as is ACC. otherwise, the DA hit is going to hit more weakly than the first (b/c it doesn't have the pDIF bonus) and miss more often (b/c it doesn't get the ACC bonus). i've never seen extensive testing showing that DAs of the first hit don't get these bonuses, but any SAM who knows the numbers will tell you that they've hit 1st hit miss + DA connecting WSs for much lower damage than they would get if the DA'd hit had a pDIF bonus.
for a multihit WS, say king's justice, you just add in the additional hits to see the % gain. for example, a 100%TP king's justice has fTP=1 on the first hit, so a DA will give you 1 divided by 3 = 33.3%~ damage. to find the % increase from 5% DA, you'd then take into account that you can DA on the first two hits and then multiply the chances of DAing by the % increase you get from DAing to find your damage. it might be because i'm drunk, but at the moment, factoring in the 2 times to DA part is beyond my effort capacity.
as for only being able to DA twice, that's been well tested on ffxionline. some guy did a ton of multihit WS, and figured out how many 0/1/2/3/4/etc hits he should get if you can DA on all hits, 2 hits, 1 hit, etc. after doing enough, you could see that it must be the case that you can only DA twice.
the goodness of DA does not depend on whether your weapon has high or low base damage... it's X% increase, whether it's increasing a number like 100 or 1000. it is true that 1hand weapons benefit more from base damage increases, so in that sense DA's value might go down a bit (because you may be comparing some amount of DA to, say, some amount of STR, which might raise base damage; if base damage is more powerful for little weapons, DA is ipso facto less valuable relative to STR, though of course a given amount of DA could still beat an insufficient amount of STR). however, it's fTP, pDIF/ACC bonuses on first hit, and number of hits in a WS that mainly influence DA's value. basically, DA does less with huge fTP or lots of native hits in the WS.
edit: typos and such. Edited, Apr 4th 2010 1:53am by milich