Ok, I understand about how combat effects influence the proc rate.
What about accuracy? I have read it make you hit more often. Then why have accuracy & Attack on the same item? Why have accuracy & attack do the same thing? Or am I just confused?
Accuracy improves chance to hit. Roughly 10 accuracy to 1% increase in hit rate (at level 70, versus a level 70 from behind).
Attack improves the damage of a hit, it does not affect hit rates. Specifically, more attack will cause you to hit for your max hit more often, compared to the target's AC. On most mobs it will have diminishing returns pass about 2200 attack, as AC values on mobs hasn't increased much since PoP.
Combat Effects are + chance to Proc effects, stacks with weapon innate +Proc mods and Weapon Affinity, increasing a weapons PPM (procs per minute).
Most high end main tanks want as MUCH +Shielding and +Combat Effects they can get, since that allows the to proc aggro/hate inducing items more often, and the shielding helps in overall reduction of damage taken. There is a cap on both of those, not exactly sure of it however, since my main is now a cleric. Please correct me if I am mistaken.
Keep in mind, procs have very different inniate, well, "Combat Effects". Some have as low at the 60's, where +100 Combat Effects will well over double their proc rate. Others have a proc rate as high as 400 (Perhaps more? I haven't kept up), where the cap of 100 CE would only give them a +20% chance to proc.