Breastplate is good even when you can't max its dex cap. Sentinel is only 1 lv 2 feat, after all. Not really what you mean, but I've never considered anything lighter.
Upon further thought, I'd say that any "situational" armors like armored coats are simply not worth it since armor runes take up so much of your budget already. Having a backup for niche situations is just not economically feasible, and rolling with worse armor all the time just invites crits your way.
It's entirely campaign dependent. If you're in a campaign where the challenges are overwhelmingly PL-1 or greater enemies on standard flat, dry surfaces, there's no reason to play with niche gear.
But the rule books are big toolboxes, meant to provide the tools necessary to run any kind of campaign you want. One that involves hanging out on a lot of ledges? Squeezing through narrow passageways? Dealing with geographical Hazards during combat (note to self: run more combats on ice)? Fleeing from enemies regularly? Then yeah, having that extra DEX on your skill rolls might be more valuable to you.
Even then though, flexible only matters if you aren't meeting the strength requirement of your armour. So it's niche on top of niche. It's crazy to me that they even consider offering people the option of +1AC for flexible. It's a very weak trait. Something that at least offered some alternative damage mitigation, like say a small flat amount of resistance to energy damage, that might be worth sacrificing a point of AC. But I just don't see it for a trait which is likely doing nothing if you have even a little bit of strength.