Sometimes, Healer and Caster gear will be able to be used interchangeably. In fact, in some cases, that is true right now. The Healing necklace from Akil'zon in Zul'Aman is, in most cases, better for casters than the caster necklace from Zul'Jin. It's a marginal difference, but a difference nonetheless. Another example of this is the healing cloak from Illidan. Healers want it, but so do certain caster classes.
This hitch in itemization is probably one of the reasons that they're moving forward with an attempt at homogenizing this loot, alongside the potential benefits for Hybrids. The change to itemization that allowed Healers to gain Spell Damage from Healing was predicted by most to not have an effect on casters whatsoever, but when combined with things like Haste and Crit, the Healing items weren't just an alternative option... they were downright better options.
Saying 'to hell with it' and combining Healing and Damage into Spellpower to homogenize the loot is a fairly decent idea. It eliminates the problem by recognizing and embracing the problem... thus, it isn't a problem anymore. It's a feature. However, this calls a few other things into play. All of the other stats.
Spirit is gaining an increase in usefulness for all classes, so that isn't a deciding factor. Haste is good for everyone. Crit is good for everyone, and MP5 is... well, eh. Who knows with the Spirit change. It might be good, it might not be so good. So far, things are alright. What about the single most important caster stat (for most)? What about Spell Hit?
We have a few scenarios here. One: Spell Hit is removed from gear, and is unnecessary for PvE/Raiding. Two: Spell Hit is kept as an essential stat, and this single stat keeps the tenuous divide between gear that is useful for Casters and gear that is useful for Healers, largely nullifying any change they've made. Three, they vastly lower how much Spell Hit you need in the endgame, and let talents handle it.
The first one is highly unlikely, the second one seems the most likely, and the third is possible. In the third scenario, if they greatly decrease the needed hit, the change to a Shadow Priest's Shadow Focus (lowered to 3 points, 1% hit per point along with spell cost reduction) would be perfectly in tune with that. If you needed, say, 1% hit per level above you rather than 5%, three points in Shadow Focus would more or less have you on the right path. Sharing gear with the healers would be easy. It's a possible scenario, one they could do, but I don't think it's one we'll see. Especially since this hasn't been reflected in any other class yet.
I simply can't see them going through with the first scenario.
The second scenario is the most likely. Keep Spell Hit as an essential stat. Caster drops and Healer drops are, for the most part, separate from one another until you're hitcapped (which is becoming harder for some in Wrath). This means that Healers can use their healing gear if they choose to respec to a DPS spec, but it's never going to be an optimal choice. To perform to the best of your abilities, Hybrids would still need two nearly totally different sets for each of their specs.
In that scenario, this change does little to help the players that want to push their gear as far as they can. It does little to help the Hybrids that want to perform their very best. It helps the players that don't care about that and only want to do daily quests, but there is a similar system in place right now. In this situation, Caster and Healer gear is homogenized... except that it isn't, and in certain cases (being hit capped), the 'Healer' gear will be better due to item budget being used better. It just won't be called Healer gear anymore.
In other words... until we learn what they're doing with Spell Hit, we don't really know what this will do for most Hybrids. In the most likely scenario, this won't change anything at all, and Hybrids really trying to push themselves will need multiple separate sets anyway, and traditional casters will be doing the exact same thing they were before. Gearing with their own gear primarily, and then picking up certain pieces of leftover Healing gear because it's better. It's not a problem, though. It's a feature.
The only people this could actually benefit is the Healers themselves. Not the Casters, not the Hybrids, but the Healing specced Healers that don't intend to spec anything else. They'll have more +Damage for doing their dailies, and that's the extent of it. To do anything substantial as DPS (dungeons, raids), they'll still need another set.
Much of this can be changed one way or another with a few pieces of currently unknown information, so let's sit tight and see what we can see, eh?