Your analysis is correct, but when considering ratings you really don't want to be anywhere near the point where you are generating 30% or 50% from rating alone. The hard cap is an absolute value that requires an infinite amount of rating to produce the max percentage. The confusion comes in when, as you say, people mix up the hard cap from rating with the suggested value on your character sheet. It's interesting how the guideline total value for defense on a jugg, for example, is around 30% while the max percentage from rating is also 30%; not sure if this was planned or it just turned out that way.
But even the soift caps are not possible to reach right now, meaning no matter what defensive stat you stack you will never get diminishing returns. So it just comes down to preference, youre too spikey? up your shield chance. taking too much overall dmg? up your def and abs.
On my gaurdian personaly(effective numbers) i stick with 50% shield , def atleast 30% the rest absorb. i dont use augs with with endurance because i find 24k more than enough hp (and ive played tanks long enough to know hp runs out and mitigation doesnt). I may up my shield chance at a later date when getting to a number like 55 or 60% is viable without gimping your other stats.