Poison doesn’t affect wights as much as I’ve noticed but specter coating works very well against them. I’ve only used it when I’m wandering in the Mounds where there’s too much annoying wights always aggroing you, while all you want is enjoying the constant rain and glooming light so you can really work on being truly depressed.
If Funcom applied your suggestion (I’ve made numerous similar suggestions) to Spectral Coating AND Poisons, by just increasing the maximum total stack number even if meant consuming 2-3 doses at once, I’d be all for it