I've got a Guardian tank, but my Shadow is my main so I don't focus as heavily on gearing her perfectly as I do my main. As such, I do actually try to follow the general gist of the optimal weights, mainly because it's true that defense is substantially useful to a Guardian than Shield/Abs.

When calculating spikiness, you first have to look at each of the comparative chances of mitigation: with 29/50/50, you've got a 29% chance of taking no damage, a 35.5% chance of taking half damage, and a 35.5% chance of taking full damage; with 30/45/45, you've got a 33% chance of taking no damage, 30.15% chance of taking half damage, and a 36.85% chance of taking full damage. Since your DR should be completely static (same talents and same armor rating), the only difference in spikiness arrives from the difference in those percentages (Shadows have higher spikiness than either of the other tanks specifically because they have *way* lower DR). You're only 1.35% more likely to take an unmitigated hit so you're *slightly* spikier, but it's such a small amount that its not likely to be noticeable. In the same vein, you'll end up taking less damage over the entire course of a fight: the first setup would have you take ~53.25% damage pre-DR; the second would have you take ~50.42% damage pre-DR. That nearly 3% difference is pretty massive, so you're trading off a bit of spikiness for a pretty large increase in total mitigation.

The reason that Guardians care so little for Shield/Abs is pretty simple actually: Shield and Absorb are synergistically useful. To make Shield useful as a mitigation stat, you need a lot of Absorb; to make Absorb useful as a mitigation stat, you need a lot of Shield. Shadows and VGs both get plenty of Shield and Absorb from their respective tank specs. Guardians, on the other hand, do not. In fact, they get a lot of Defense, which ends up just making Defense even more valuable (the higher a mitigation stat gets as a percentage, the more valuable from a comparative mitigation standpoint further increases get). Once you realize that, it becomes pretty obvious that Defense stacking is the way to go.

I'm finding it very hard to match the numbers, I'm trying to lower my shield/abs evenly to those levels, absorb I can drop easily, but shield comes with every piece of enhancement so it seems like I will always have more shield than absorb. I'm at roughly 656/689/387 (def/shield/abs). How can I swap shield for defense? Is there a workaround I'm not seeing?