The OP is making a mathematical argument, that a lot of people are missing. Let's take the words "druid", "shaman", and "paladin out of the conversation for a second. Let's just imagine an abstract situation. You have a stat budget, let's call that quantity B. You can assign points to attributes A1, A2, ..., An. Each attribute has a cost modifier C1, C2, ..., Cn such that the budget for an item can be written as the following bound:

Sum{i = 1 .. n} ((Ci * Ai) ^ (3/2)) <= B

Now imagine you actually have 2 sets of items itemized with this basic constraint. Which is a better stat assignment strategy? Purchase many stats in smaller quantities or a few stats in large quantities?

Ultimately, the answer is many stats in small quantities. In less abstract terms it's better for a lowbie to go out and acquire a bunch of "Of the Monkey" gear than it is to acquire a bunch of "Of Stamina" and "Of Agility" gear - he'll have more total stam and agi via the "Of the Monkey" gear than he could ever have focusing on single stat gear.

However, this result is just as true for 3-stat and 4-stat pieces (i.e. pieces with 3 and 4 stats on them) as it is for 1-stat and 2-stat pieces. Given the choice (and assuming that you can get the stats you need) it'd be better for every single piece of your gear to spend a little of its budget on every stat worthwhile for your role than to spend all of its budget on just a couple. Hypothetical items that spread stats across 8 or 9 stats (assuming a sufficiently large budget) would be inherently superior because of this.

What the OP is asking is, is this effect actually significant in the current game? Can, say, a druid acquire enough decent gear with stats spread across all of the various useful stats to be able to do a satisfactory job at most roles while being some reasonable %age of the effectiveness of someone who is more specialized? (All without giving up effectiveness at your other abilities.) Theoretically, a druid who geared this way would be able to heal pretty well, tank pretty well, and DPS pretty well in one set of gear. (But he wouldn't get the same level of efficiency in any of those roles versus a gear min/max'er with present item budgets.)

In the current game, I don't think the gear exists for this to happpen. It's unclear whether this potential scenario even crops up, given the itemization we see and the various other complicating factors. But, mathematically speaking, there are distinct item budget advantages to spread stats around heavily, and there may be a a way to optimize your gearing to take advantage of this.

To be clear, someone itemizing this way would never likely approach the effectiveness of someone gearing for a specific role. Even in a given role, there are many stats to spread across, and the exponential cost hasn't kicked in to the point where it's cheaper to put (say) 15 int on a breastplate than it is to add an extra point of strength. But if (when) you reach that point, this is a perfectly reasonable question to propose.

(If I had more time I'd actually dissect the question mathematically and figure out what the actually general answer is. But I don't have the time to work the equations.)

EDIT: cleaned some stuff up and expanded on my rambling a bit.

The OP's post, actually can be viewed in a non-hybrid context, even. Consider warrior gear. Warriors can get DPS plate and Tanking plate. One could ask the question, though, is there some middle ground (non-existent, but possible under item budget rules) gear that would allow warriors to tank and DPS at some relatively large %age of effectiveness of someone in specialized gear. Imagine plate with basically all of the following stats on it, but in smaller than normal amounts:

Armor
Strength
Agility
Stamina
Attack Power
Hit
Crit
Weapon Skill
Dodge
Block
Defense
Parry

Since you'd be investing in so many stats, the point totals would be low. But you'd get more than if you mixed and matched DPS pieces and Tanking pieces.
The interesting question is if there's a breakeven (or better) point for gear of this sort. (I don't know the answer though)

You see this question coming up in the context of hybrids more often because the range of potential stats they can benefit from is larger still. A druid can be concerned with all those stats and then:

Int
Spi
+dmg (and its variants)
+heal
+spell crit
+spell hit
+mp5
-resist

(and possibly others.)

There's actually an intereting optimization problem underlying all this for gearing. (And I'm somewhat hoping someone with more free time will ponder it...)

Unfortunately, the item budget math disagrees with you in general. At some point the cost of specialization exceeds the value in diversity. We all know that the morons who go out and wear all "Of Frozen Wrath" cloth, or "Of Stamina/Agility" are wasting valuable stat budget points that are better spent on gear with 2 or 3 different stats on them (e.g. "Of the Monkey/Bear/Eagle/Whatever"). It's just less clear (and harder to find nice "peaks") when you're dealing with far more distributed stats.

The real question is: Is there some point where more hybridized gear is nearly as good (at a given job) as specialized gear. To be clear, I don't think the items we're seeing right now have quite large enough item budgets for this sort of thing. But their is an exponential cost on growing along any single stat axis, and that encourages stat diversification innately as an optimization strategy.

I'm not saying that it's smart to go get a weird mish-mosh of gear (it's not), but that it should be possible to devise extremely broad (stat-wise) sets that get very very close to the effectiveness of all the various rolls they can use at one time. That being said, I don't think such gear exists. But the stat budgets do not explicitly forbid them from existing. (Unless there are additional stat budget factors that we are not aware of...)

Yes you do. There can't be a point at which an item with less stats allocated to stamina, defense, and armor is a better tanking item than one with more. If you want to come up with some metric or heuristic that gives you a number for how much Hybridy Goodness an item has, by all means do it (though maybe not here) - but you haven't solved an interesting problem, you've just come up with a way to produce numbers that aren't relevant to any in-game scenario.

Think it over time. Come back, one week.