Supposedly they got rid of heal spells to open up party diversity or, at least, that was one of the major reasons. That doesn't look like it will be the case in practice, though. I wonder if they (Bware) have gone through all their videos/streams so far and noticed how much they speak to the necessity of the Barrier spell in ~every single one of them~. So, nothing's really changed, except now you're mitigating dmg instead of healing it. Should have just let us do both and pleased everyone involved but, for whatever reason, they didn't feel that necessary.
Other classes have the ability to create barriers as well (such as Guard for warriors) but the big difference is that those are more for self-sufficiency. From what they've shown so far, Mages are the only one that can protect the entire team, and their Barriers are what was shown keeping everyone alive even with Guard and such, which didn't seem to hold up very long at all; so many times in the Twitch streams, the boss swings his weapon once and all of Cassandra's Guard + half her health disappears instantly. What. Ever.