A few days ago I happened to inspect another paladin in Orgrimmar and noticed that her gear was... odd. It was a mix of holy, ret, and prot gear. If you check my armory I might be wearing a prot chest and PvP boots and trinket in what is otherwise a decent ret DPS set, so I would seem to be a hypocrite for seeming critical of the person in Orgrimmar. Maybe I am! But there is one thing which I think stands out: enchants and gems. I am clearly wearing a tanking chest and PvP boots and they are gemmed and enchanted as such, so presumably I know they they don't belong. The paladin I ran into had the LW AP 'enchant' on her wrists, which were holy wrists.
This seems pretty stupid, right? Perhaps. But I try to avoid quickly dismissing people as stupid. Instead I believe the person in question may have not had the idea that gear is designed for certain roles. That sounds obvious, but is it really? The holy bracers had crit, SP, and intellect on them along with stamina which doesn't really differentiate anything one way or another since it's on everything. The crit, since it was unified with the introduction of 3.0 (or was it 2.0?) is definitely useful for DPS. Spell power is a weak stat, but someone could honestly and accurately claim that is is a beneficial stat, since many of our DPS abilities do scale with spell power. We even have a talent to increase it, so it's not so ridiculous to assume that spell power is a good ret stat.
Experienced players automatically do a lot of meta-gaming, trying to figure out what the devs intend, so as to gain an advantage. To us it seems perfectly logical that the devs would break gear into five different categories: DPS, tanking, healing, PvP, and cosmetic. So of course we'd see spell power and put it into casting: DPS or healing, and then judge that since they are plate they would be intended for paladin/warrior/DK, see that none of caster DPS trees, and reach the logical conclusion that they are for holy paladins: healing.
Imagine if you didn't break gear into those categories automatically but instead were shown gear and asked to determine if it is an upgrade. That would be a much more difficult task because you must evaluate a much larger pool of gear rather than being able to automatically throw away two-thirds of the possibilities. As with so many automatic human processes, these save time but not perfect; they are effectively a variation of stereotyping and can cause upgrades to escape us because they're not supposed to be DPS gear.
Oh there I went rambling on and on... When did you start calling some items healing gear and other items DPS gear and still others DPS? Did someone tell you one day when you rolled need on the wrong type of gear or did you figure it out on your own? Perhaps there was some guide which explained it? I've never seen such thing, but I don't see why it wouldn't exist. Perhaps it should if it doesn't!