Assassin's Mark, Guardians of Empyrion, skill assignment and player buffs

If i assign Assassin’s Mark on Guardians of Empyrion, doesnt it become worse because they wont benefit of Fighting Spirit’s OA buff? Wouldnt it be better to assign it to a player skill then?
Or how is this working?

Edit: Dam i just realized that just started 2 post with the same kind of problem.