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So, in the best case scenario when casting it on a Rogue (with the assumption that poisons can't crit) is that it will be half as effective. Still worth casting, imo. It would probably be most useful on a Rogue if the only other option is, say, a Tree. Even if it does give them 3% crit, they probably don't need it. And a Rogue will guarantee the uptime on you. :D
If you have a caster to put it on, that will offer the best net DpS gain for the group (at least in most situations). But I can see why you'd put it on a Rogue in at least a few likely situations (A group with only you and a Resto Druid as casters isn't that unlikely).
Excuse my lack of knowledge but is there a reason for not putting focus magic on a tree? I tend to use mine on healers more often then DPS.