A better rule is: Always go for Haste/Mastery/Accuracy gear. Failing that, go for Haste/Crit. Avoidance gear is only useful if it is a marked increase in ilvl (i.e. 13+), else you're better off keeping an item that is better itemized.
Example (happened to me):
I had a Exp/Mastery helmet from t14H (509). It's not the best itemized (no haste) but it's not terrible.
A 522 Dark Animus helmet drops (haste/crit). Me and the Ret/DPS DK both can use it. Personally, I'd pass it to the Ret/DPS DK (unless they're going to use tier helm). IF they're going to use tier, it's likely better for you to take it, as it's a really solid helm. I passed it to DPS.
A 535 Horridon helm drops (dodge/parry). It's shitty itemization, but it's a 26 ilvl upgrade over what I'm using, so I take it.
Eventually I'll move to HC tier or HC Animus helm, whichever drops first.
Since then, I got a 522 Animus helm, but don't use it over the 535 D/P helm. The haste is great, but the crit holds it back some. Add to that, it's lower ilvl and I ended up just sticking with the Horridon helm.
Obviously, everyone's opinions will vary and differ based on loot luck, comp, and preference. And that's exactly why you can't make sweeping generalizations about what is or is not better for everyone's raid.