Keep in mind, Parry is a binary result. You either Parry, or you don't.
Lets say during this 10 second duration you take 5 boss swings (seems likely since 2 second swing timer is quite common). You need to take 200 swings before that 0.5% parry gained actually gives you 1 additional parry. This means you had to use it 40 times (40 minutes) to get the benefit of one additional parry. That's just a statistical average. On a 10 minute boss fight you would expect to gain one quarter of a parry, or zero parries.
Deflection Plates is 12 seconds so you get 6 attacks worth based on the same 2 second swing timer model. Of those 6 swings, most of them will not be dodged, parried, or missed (most will be hit or block). Lets use a very generous avoidance and say you avoid 50%, so you get hit 3 times each time this buff is used. On a 10 minute boss fight this ends up being 30 hits that were mitigated slightly higher because of the higher mitigation. Those 30 hits partially reduced means you took off 30% of a single hit's damage (if that was 1% armor). In practice since your avoidance isn't as insane as 50%, this will actually be much higher.
Now, because of Hold the Line, this 30% reduction of a single attack's damage versus the 25% chance of fully avoiding one and gaining the benefit of Hold the Line these two may come out to statistically be very close in value. However, you have to always keep in mind, until you actually get that additional attack parried, you gained nothing from Synapse Springs. You need a 20 minute fight before you expect (with rounding) for one additional parry to occur. Since we don't have 20 minute fights, this actually is worse even if statistics may say otherwise (because many statistics are run over infinite time, fights don't actually last forever though). On the other side you are guarenteed to gain something from Deflection Plates.
I hope this sheds some light on it for you.
Last edited by Quinafoi; 06-08-2011 at 12:15 PM.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.