Why was Bruce Wayne so scared of these things?
I got my priest some heirloom gear, which I thought meant I wouldn't need to replace her gear till level 80. But here she is, level 34, and I get a shoulder that increases her stats significantly! Am I missing something? If I have the heirloom gear in my bag but not equipped, do I still get the xp bonus?
The general idea is that even if a piece of gear has better stats than the heirloom gear of the same slot, you'll still level faster and more efficiently with the heirloom gear. A stat increase would have to be absolutely enormous to compete with an XP gain bonus-- far more than a single slot could contribute.
And no, you have to wear the heirloom gear to get the bonus. It doesn't work from your bags.