A tank's primary roles are threat control and enemy positioning. You need to be able to generate a lot of threat on yourself to get enemies to ignore your DPS/Control/Support characters, and do so reliably. Yes, since you are drawing a lot of attention on yourself, you need to have strong defenses.
Pull of the Abyss is a Control skill (a.k.a. Crowd-Control), not a taunt, and Spirit Blade is a DPS skill. Fade Shield just gives him good defense. The fact that the AW's defense relies on him doing damage (a la DPS) should make things pretty clear.
Regardless, I can see the need for the AW to be the de-facto tank if a real tank is unavailable, but directly interfering with a tank's job is my annoyance. I play a tank a lot, and I do it well. If I am in the party as a tank (or anyone else is a tank, for that matter), the AW should not be trying to tank, but rather being the DPS/Control that he is supposed to be. Pick your choice of his real skills: Inhibit, debilitate, debuff, destroy... the AW just needs to leave the tanking to the actual tanks.
Yeah, I'm one of the few that would have no issues with Fade Shield being nerfed, say, to only generate barrier upon successfully deflecting with Spirit Blade. Something that still gives it purpose, but stops confusing class roles for players. The Arcane Warrior in Origins could tank, but that was because threat could be generated reliably by wearing massive armour and Fade Walker was an amazing sustained ability for defense. The Arcane Warrior here is by no means what it was in Origins, and I notice many almost seem to want to force it to be.
Why are you defining something by what it
can't do instead of what it can? "The Arcane Warrior has to attack to get threat/barrier, so it can't be a tank". So freaking what? Fact is he can generate enough threat to gather enemies and enough Barrier to take the hits, noticeably whenever he wishes due to spirit blade being cooldownless and cheap, which the Lego(or anyone really) can't do with their far inferior guard.
Just because the Arcane Warrior isn't rocking full plate mail and a tower shield doesn't mean he can't tank. Instead of telling players they're confused about what roles each class are the intended spiritual successors of you should realize what classes can and can't do in the current meta, because that's all we got right now, for better or worse. Perhaps the AW's Barrier generation will be nerfed in the future and he won't be able to tank, heck I'd welcome it, but for now AW's stealing your Lego's threat is not a problem.
I agree with both rat and penguin on this. The Keeper doesn't lend itself to DPS, it's an almost pure support class. It's not mandatory - no-one is forcing you to take barrier - but it can really help, especially with weaker teams that need someone to take care of them. It doesn't have to be taken, but it definitely gives you alot more flexibility, which is IMO worth it. For me, it's more the fact that there's no reason to not take barrier as a Keeper.
I'd also say there honestly aren't so many skills in contention to justify skipping barrier on a Keeper in the first place. Chain lightning / Frost Step or Fade Cloak / Static Cage or Disruption Field / Barrier is a pretty standard build. If you need another slot, it's infinitely better - imo - to lose chain lightning than to lose Barrier. Especially considering most of an entire tree is dedicated to or work well as barrier passives which would be wasted if you don't use barrier.
What did they say during the pre-launch MP video?
"Oh, we made sure each class had a few viable builds. We're playing Keeper with barrier now, but you could totally play a barrierless Keeper and not miss a beat."