I have been debating this one for a long time.
Most peeps seem to think that having a damage proc is the way to go, but because i am a parse Nazi (ie i am always running the parser), as I imagine most scouts would be, i have noticed a quite peculiar trend as I analyze the parses at the end of the nite in my quest to maximize my dps and determine why some scouts are doing more or less damage then me.
What I discovered is that on the zone wide parses, that majority of the raid, including all scouts (melee scouts), were only averaging a 80-82% to hit rate. This is amazingly low. Now this information is not from just one raid. To ensure accuracy and a good selection I have been examining the data of parses for a couple months now from EOF RAID zones. The results are always the same. I even contacted some rogue buddies in other guilds to have them check their zone wides and they didnt believe it either.
So what I concluded from this data is that it would be much better to get a +5 pierce/slash adornment for your weapon than a proc that only procs ONCE per minute. The damage of ONE extra auto attack is way more than one proc. Additionally, the chances are that you will actually hit more than just one more time as you increase the skill level.
Now with that said, this is all basically theory.
Thou i did finally decide to make myself one tonite, as i have been trying to hold out for the AoNO to drop but it looks like its gonna be a while, so i finally decided to adorn my CDS.
Normally I have approx 429 piercing, well tonite I had a bit of a group change, as normaly I am stuck in the mage group! Well tonite I was in a different group and with the buffs I was getting my piercing was 480. I noticed immediately that my DPS had increased dramatically. In fact some peeps were sending me tells cause i kept topping the parse here and there, which normally doesnt happen when I am in the mage group
As I analyzed the parses after the raid (EH). And noticed that while i was in the group in which I had 480 piercing, my to hit % was 87%. A 7% increase, and by the end of the nite when the groups got reworked for a specific fight, and I ended up in the mage group again, and therefore my piercing dropped back down to 429, the parses showed that my to hit rate also dropped back down to 82%. Wish I had the parse still open to submit the data on how much more melee damage I was doing from the additional 7% and how many more hits that equated too, but already closed the parser. maybe tomorrow.
The bottom line:
I think that any + piercing/slashing skill items are a worth while investment and I think this may suggest that even thou people believe that there is a cap, which i am not sure there is or that there are diminishing returns for it. I think this proves that increasing your skill above the proposed cap still is quite effective.
disclaimer: Unclear what the delta would have been if I had just had the +5 adorn itself without the buffs from this new group setup, but from knowing that my to hit rate was usually around 79 to 80% during previous raids, I can surmise that the +5 piercing may have raised my to hit rate by about 1-2%, which I think is still quite significant.