Imaro
Legend
Which really speaks to Imaro's point that the paladin is gaining advantages by not following his code. In actual fact, poison instead of other more appropriate consumables is actually not more powerful and may well be less effective.
For example, anything with a burst effect would help his marking abilities.
What other more appropriate consumables within the same level range and price point are better for the paladin? I'm also curious how those with a burst effect would help his marking abilities since it isn't actually an attack and only targets one creature? It's all well and good to claim that something could be better suited to the paladin, but if this better item doesn't actually exist, well it's a moot point then, right?
Finally... neither you nor @pemerton has shown why the poison would put the paladin at a disadvantage, in the worst situation he gains no advantage from it (when hitting something like a minion, of course the poison works like that for everyone so it's a "disadvantage"shared by all)... in the best of situations he delivers an extra ongoing 5 poison damage that a save ends... mechanically I still haven't seen a reason that I would not want that advantage.
EDIT: And saying, that he could benefit from some hypothetical consumable that is better suited but does not in fact exist at the same price point is not proving anything.