looks like the ac stacking issue only affects self ac buffs, not all ac buffs. since shamans dont get the int caster sheilding line of spells, this isnt a huge issue. the only relevant stacking issues then, are things like illusion ac buffs and DSs with an ac component, as outlined in the OP. Since you dont normally get these spells "thrown" on you in the heat of battle when you would need slumber, i dont see that as a relevant argument for change.
Since this spell gives resists, ac, and a larger hp/tick heal, it IS an upgrade. the reason torpor doesnt have the same stacking issues is because it has no ac component, as stated by Tyrone.
Changing this shouldnt be terribly difficult, but ill leave it to the devs to decide if its intended or not, and can be changed.