I don't see why you wouldn't just sacrifice AR unless you absolutely do not need more vitality. Frankly, the Diamond is way disproportionate compared to other gems when compared to AR vs. primary stats on gear. For example, I think my Cindercoat is something like 460 dexterity and 90 AR. As I said above, it doesn't even take two
free gems to recoup that 90 AR, but it takes far more to get the dexterity. Although, I took a gander, and while the gems start at the same resistance value, the highest level diamond is only 74 AR vs. 280 primary.
Also, what about my notion of a logical "soft cap" on resistances? Resistances seem to have a somewhat logarithmic curve, and there's a point where you can gain significant amounts of resistance points to see very little mitigation. Now, I don't have great numbers at my disposal since most of my "findings" are just from watching my detailed character pane while adding and removing gear. However, on my Crusader, my resistances vary from around 230 to 650. So, I did things like remove some gear to take one that's around 360 to 230 and see how much I gain, and I did something similar to take the 650 down to 530. I think the jump from 230-360 was something like a +10% gain and 530 to 650 was only 2-3%. They were both very similar amounts being added that saw great differences in their effectiveness. Albeit, I think I'm around 70% when I'm at 530 resistance!
One other thing that seems weird to a WoW player like myself is that it seems that you can dodge
any projectile whether it's an arrow or a magic spell.
I don't really have many hard numbers for this (oh, what I would give for an EnhSim for Diablo 3!), but I'm pretty sure that I've seen some stats that you can also roll on gear that are more powerful than your primary stat. Of course, I'm talking things like +100% critical strike damage.

I guess what I'm trying to say is that there might be instances where +450 primary may not be better than something else that you could roll.