Ghoulbane and Staff of Undead Legions are nice for undead.
I did some log crunching on 2nd floor CoM at some point with just the staff and concluded it improved your DPS by about 15% overall (taking into account that all mobs aren't undead - staff only procs on undead).
Overall exp gain due to staff was probably significantly lower because you still get charm breaks and need to run around to mobs, and that time spent is unaffected by the staff.
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