Except that that uses Mana instead of Rage and it wastes a cast turn, which in alot of cases is more worth than a Chest turn. Not always useful but sometimes you just need to stop one stack from advancing.

That's true, a spell could come more in handy than a rage ability at times but making for instance a phantom and placing it within reach will more often than not soak a lot more than just one hit of damage, even more so if you make some thorns which cost no mana and are usually a high priority for enemy attacks. But I guess that's just from a warrior's perspective, I rely quite heavily on rage abilities and less on spells which are few and weak.

the AI sometimes destroys it, sometimes walks around. I find I very very rarely have use for it. But when I do it is a lifesaver.
In the right type of map, with natural choke points, I can use it to waste the turn of SEVERAL enemies. (by using them to block their comrades.

I believe that the AI logic in questions of walls is very direct/clear:

If there is a way around - they take it...

If AI has more units - it may calculate if it is more effective to lose one turn of one unit to destroy it - opening so the free way to others (I write "it may calculate" - as often I see that I would surely rather destroy the wall instead of going around with each of mine units...)

If the way around is longer that losing one turn on destroying the wall - AI will more positively destroy it...

Still I found that sometimes AI get stuck when trying to overcome the wall

If there is totally no way around the wall - the wall get's destroyed for 100%