Shamans who have a Mantle of the Forest would get more AC from spikecoat than they would from slumber. Furthermore, the AC benefits of slumber last for like 4 ticks as opposed to the 1200 ticks from spikecoat. It just seems that slumber should be able to stack with the AC buffs like torpor, or remove the AC part and add more healing.
I'd Rather see Slumber of the Beast turned into a targeted HoT *shrug*
But I do see his point about the AC stacking issue. Does Slumber at least Override the other AC buffs? Because if it doesn't that could really suck if you dont notice if someone had thrown a AC buff on you and you really need slumber up.
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.