SAM/WAR can tank just fine, but really only shines as a tank when you merit Third Eye recast and your Parrying Skill reaches its maximum. Unfortunately, people are often unable to think outside the box, and they always merit only Meditate and Store TP on SAM. Also, skilling-up Parrying Skill is a pain in the butt, and (unless I've not caught on to an efficient way to skill it up) laboriously slow.
If you have tons of support as a SAM tank, just use Hasso and damage-deal like an animal in-between Provokes. If you lack for support, use Seigan/Third Eye, and swap-in Damage-reduction equipment when your Third Eye drops.

Ninja is still likely to always be a better tank if the mob uses single-target nukes and/or multiple Utsusemi shadows are able to block some of the mob's nastier TP moves. You could do SAM/NIN, but that's a subpar job combination in my opinion. Having either Hasso or Seigan up will gimp your recast timers on Utsusemi,. Your damage output is gimped on SAM/NIN by lacking Berserk, Warcry, and Attack Bonus. Also, NIN simply gets more shadows per cast.

Bottom line, SAM can definitely tank, but it's typically not the best option. Personally, I think it's best to carry your tanking equipment on SAM at all times, just in case it's needed. However, despite my preparedness, I don't actually end up tanking on SAM very often. If you want to tank "right" on SAM, as I stated before, you gotta level up Parrying Skill and merit Third Eye, in my opinion.