One of the exciting announcements that came about with Legion was that switching between any of your class' specs would be doable out in the field. Instead of going back to their trainer to respec and having to set up their bars all over again, you'll now essentially be tri-specced (or quad-specced for druids and dual-specced for Demon Hunters.)
That sounded pretty great for people who wanted to dabble in all their class' specs, but there were a couple issues. The first was the new artifact weapon system. We're going to be spending a bunch of time working on accruing artifact power to fill out the artifact "talent" trees. And since each artifact is limited to a single spec (you can't use your Arcane staff if you switch to Frost, for example,) that means that leveling up multiple artifacts might require a great deal more effort.
But the Alpha has revealed some additional hurdles.
The first one is that, as far as I know, there's nothing in the current build that allows you to do a different artifact quest after you've chosen your first. That means that, at this stage in the Alpha, you need to be very certain which spec you want to play.
I'm sure that they're going to provide some option to change your mind, as this would be particularly draconian. But I'm not entirely sure that we'll be able to get additional artifacts before level 110.
The other major issue added is that in the latest build, there is a gold cost every time that you switch specs.
Now, to be fair, it's 100 gold, and while that may have seemed like a lot of gold back when the only way to change specs was going to the class trainer (and I think that plateaued at 55 or something,) these days 100 gold is probably less than you'll get for doing a dungeon run.
But the signal that this (potential) change gives is that Blizzard really wants you to choose a spec and stick with it. And I can respect that that would seem to make the choice of spec a more significant one - one that allows you to really identify with that aspect of your class.
Of course, I'd argue that dual-speccing never broke that. Yes, I might solo as Retribution on my Paladin, but if someone ever asks me what my main is, I say a Protection Paladin. I only rarely run any instance that requires any tanks if I'm not tanking. Even for pure dps classes that can use the same sort of gear, I still think of my Mage as emphatically Frost, and I don't have enough mastery to make his Arcane spec really work.
It's still early (though I'd say testing has exited the "super early" phase, even if they're still calling it an Alpha,) but I think that if this is the direction that Blizzard wants to take things, they need to work on existing issues with solo tank and healer play.
I can speak a lot more about tanking. Tanks are good at surviving. They basically play to try to make it take longer for enemies to kill them. But how should that translate to solo gameplay? Back in the day, you could kind of enjoy less downtime because you wouldn't take as much damage as DPS players. But these days most DPS specs have mechanics to heal them up after killing a hostile creature.
Give tanks more damage and DPS starts to complain that their whole reason to exist is usurped. But if you make them too self-sustaining, you run into balance problems when they really are tanking. Tanks are not built to maximize damage output, and outdoor enemies tend not to hit terribly hard, so the tank's skillset makes for pretty slow solo gameplay.
Healers also tend to only have a couple of damaging abilities - enough to let them kill an enemy, sure, but not really to have an engaging single player experience.
And that's really the crux of this. Tanking and Healing in a group setting can be a ton of fun, but in a solo environment, even if you can survive just fine while battling your foes, there's no real advantage you have over just switching to a DPS spec - DPS can survive just fine, and they can kill things faster as well.
So they either need to do something like a toggled ability that increases damage output incredibly but also prevents the player from performing their group role, or they could do the simpler thing and just let us swap out specs when we want to, just as we do now.