My point was that getting that SR has far too high an opportunity cost.
You make it sound like you pay 8 Feats to get SR 32.
You pay those Feats to get 7 other Rage effects
and SR 32. The opportunity cost of Mystic Rage is one Feat. Not eight.
It's not jaw-dropping, but to my knowledge, it's only done for purely thematic reasons. No one should go pure barb for the mechanical power when Rogue 5/Barb 15 gives you the same 20 levels of Uncanny Dodge to negate flanking*, plus all the lovely Rogue Skills and Sneak Attacks in return for a small loss of AB and HP. Dramatically more utility and survivability is gained for a small loss of specialization as a heavy-hitting brute, the same as the dilemma between going pure Fighter or Rogue 5/Fighter 15.
You're incorrect about pure Barbs losing Flanking. Barbarians get Uncanny Dodge.
The things that a Barbarian loses out for dipping 5 levels into Rogue is upgrading his DR from 3 to 5, Tireless Rage, another 2 Rage per day (in other words, 50% more Raging), and upgrading that Rage to a further +2 to STR/CON/Will while raging.
All the above are big, if not huge, opportunity costs -- especially when you take them in conjunction, since all those Rage effect boosts stack together.
Now I won't argue that Rogue 5/Barbarian 15 is bad -- any Rogue dip gets UMD so by definition it's good. But there's no comparing pure Barbarian to pure Fighter. It's an excellent class.
*I'm not 100% certain Uncanny Dodge stacks when multiclassing in the classes that have it for the purpose of preventing flanking. I recall an informal confirmation on Discord, but there's nothing official on the wiki. If it's false, that at least is a good argument for going pure barb.
Pure Barbs get Uncanny Dodge and immunity to flanking. That's why pure Barbs are so good now. You get a level 20 melee frontliner who can tank in places where mobs have sneak attack.
So: in PvE, Barbarians have an unique niche and remain a hard-hitting melee frontliner. In PvP, they have access to potentially instant +10 Strength that will make them terrifying in any up-close encounter and SR 32 which is on par with monks or level 20 spellcaster Spell Resistance (and again, they don't need to spend 6 seconds casting to activate these powers -- they activate with one Rage click). They are the PvP equivalent of high noon gunslingers, they go from standing to killing you in the space of one move.
Barbarians are not weak; they are an excellent class.