There's a chance I'll be mistaken here, so others will correct me, but it goes like this - ground AoEs like Hail or Caltrops will recalculate their stats every damage tick, so only their ticks that happen on back bar will benefit from the Hunter slotted on back bar. Single target DoTs though - that being Poison Injection - will take 'snapshot' of stats to use throughout the DoT. I frankly thought it was fixed (I never actually took time to check that), but I recently heard it's still the case. So, PI will run with weapon damage from back bar, and Hunter will affect it.

@Armisael9 , that ground DoTs recalculate stats for every damage tick, I think that was somewhere in patch notes from beginning of the year; I'm having trouble times finding it. That 'sticky' single target AoEs work with stats derived at the moment they were cast - someone on forums tested it, apparently, it came up in conversations, but it'd be handy to check from time to time in case ZoS made some changes (nothing about it in recent patch notes though, not that I can remember).

@witchdoctor , Evil Hunter is handy for moments when you just don't want to burn weapon power potions to get it (like between bosses, on trash fights, or on a boss you don't feel worth burning expensive potions on). Silver Leash may be of more use, but for long time it was bugged and wasn't giving weapon damage boost from FG passive (it's fixed now though). In addition, getting it passively is a bit of a boost (for instance, if one runs AY front bar, then they don't want to use more than three skills on back bar in rotation, not to let AY stacks fall off).

@SodanTok , you sure Poison Injection recalculates its damage? Hail and Caltrops yes, but I just made a quick and dirty test, and PI doesn't. And more skills on back bar isn't always good (more skills included in rotation, that is). Some sets will lose stacks if a lot of time is spent on back bar, plus the longer you spend on back bar, the more ground AoE ticks with lower stats you will have.