Reducing the importance of healing did accomplish several things:
1) There was no incentive not to get in fights in the first two Dragon Age games. You could spam potions through any battle and you would be healed up at the end. DAI gives you an incentive to avoid some fights — you don't want to run out of health potions before reaching your goal.
2) Adding Barrier and Guard does allow the devs to add insta-kill attacks, like a giant's boulder. You cannot simply heal your way through such an attack. The only way to survive — besides dodging, obviously — is to have a Barrier up so it soaks up the damage.
3) Do you remember all the people who complained about having to use Anders in DA2? They were totally wrong; the best defence in DA2 was a good offence, and Merrill provided a very good offence. But still, they complained. They complained and they complained and they complained.
Do you hear anyone complaining about having to rely on one mage over another anymore? No? That's because any mage can cast Barrier, and warriors can generate guard.
That not a problem with the barrier and guard system though, which functionally just replaces healing. Mechanically it's the same. You add health to your character to help them take additional hits.
It's more balanced than healing because theoretically you could limit extra health on vulnerable dps/control characters, and give tanks easy access to extra health.
Even in game you can see the effects on this. Guard isn't worth spit on dps characters on higher difficulties, because it's based on max health, which mages and rogues don't have an abundance of. It's much more beneficial to position so you don't get hit without barrier on.
actionhero112 speaks truth.