Threat is based on DPS: more DPS -> more threat -> holding aggro longer. There are various threat bonuses (e.g., defensive stances, racial amp, Intim, gear, etc.), but you still need DPS to hold aggro. "Intimiturtling" is a thing of the past.
So, while there may be good reasons to give up the ftr capstone, any DPS loss is
also a threat loss; that has to be factored into one's consideration when making a tank.
E.g., I'm leveling a dwarf ftr 12 / pal 6 / monk 2 build (SD II / KotC or HotD I). Pally 6 provides Divine Grace, fear & disease immunity, Divine Righteousness (which stacks w/SD defensive stance), Smite, Divine Favor, Might, Sacrifice. Monk 2 provides 2 feats (so I only lose 2 vs pure ftr), Evasion, and +3 to saves. Basically, I'm hoping the survivability bonuses from pally + monk make up for the DPS & threat loss of not having SD III + the capstone. I'm also counting on being able to go Dwarven Defender when the Enhancement overhaul is finally done so I still gain full SD benefits. We'll see how that goes...
