I'm sure any druid can tell you barkskin is no ice block, divine protection or dispersion as far as defensive CD's work go, but by no means does that make it underpowered or useless. It may only be a 20% reduction, but it takes no gcd which makes it the only defensive cd to not cause a dps loss if you time it right. It also has a shorter cd then all the previously mentioned so that makes it very versatile for fights where there are a few bursts of damage. The one problem with it is since its only 20% it cant always save you from death so knowing when to use it can be difficult, but overall I've come to enjoy using it quite liberally to save me or even to reduce the risk of death, dead dps is no dps and it is way better to know that you wasted that 1 min cd just in case instead of face first on the floor smashing your head on your keyboard saying WHY DIDN'T I JUST HIT IT. The best way to learn when to use it is to KNOW TH FIGHT MECHANICS, that is actually the best way to survive in general on any boss of any expansion. So here is a rough outline of ICC bosses and Good times to use barkskin.
If its marrowgar(assuming HM since NM is retarded easy) when would you cast it? Well if you were not paying attention or just got cornered by being in the wrong place at the wrong time you need barkskin to live through a tick or 2 of Bone Storm, but hopefully you just learn to be not bad. for LDW you would use it most likely if you get a DND cast right on top of you or maybe if you randomly were an idiot an got hit by a ghost(though out of shame you should just kill yourself anyway). With DBS you wouldn't have as much of a chance to use it unless you had the mark and got below 50% health, but if that happens that can really make the difference between a wipe and a kill.
FesterGut is a great example of where you can actually gain dps time by using Barkskin, for this fight you only need to waste your time getting two debuffs and you can neglect the third and just use barkskin on the inhale since there is no other time you could even use it really( especially since you can't cast it while disoriented with the puke). RotFace is great to us when you get the slime on you an need to run out its a little random as to how much that can help you but it really gives a bit of reaction time to the healers. Professor Putricide doesn't really have a super good time to use this besides green ooze explode or maybe on p3 if you absolutely need to run through slime to get back with the raid.
Blood Princes is another fight that really doesn't have much damage that will affect you that you cant avoid or it wont kill you(unless your randomly a tank for the ranged dude). During Blood Queen's air phase it is awsome to use this once again more of a healer helper, but if your missing health then its a must have for each air phase, you can even use it if your about to b bitten and your not at full health.
Dreamwalker really doesn't have much of a use for it at all except if your stupid enough to kill an exploding zombie right on top of you. Sindragosa really only has one use as well, for if you get ice blocked and your asphyxiating or if your about to get blocked and you aren't at full health, but you cant use it with unchained magic as it adds an extra stack thus making it counter productive XD.
Unfortunately since i just stopped raiding for a while due to a schedule change at work, I have not had the pleasure of killing arthas and therefore would not be able to tell you from experience when a good time to use Barkskin is lol, but don't worry I'll still be posting even during this break and I make sure to keep as up to date as can be as far as the moonkin business is O_O. And on a side note remember to use barkskin for predictable incoming damage, but its usually better to use a health stone for when you get taken very low since once your at low health chances are barkskin wont really save you anyway.
1.prof has a huge epeen