why should it, murrph
you are paying 75 mana to do two things
(1) convert your attack from physical damage to magical damage
(2) apply the harkon's debuff for 3 seconds
you only get (2) in the case of ability use, the ability itself does the same thing it would normally. Unless it was meant harkon's could allow physical abilities to do magical damage as well -- I didn't interpret it like this.
and arguably (1) is a much more powerful effect. Consider that if all harkon's blade did was apply (2), it could've cost no mana per attack and still be a pretty awful item - completely outdone by hellflower.
"Proclamation was made, in the King's Name, for all Persons to keep Silence" - House of Lords Journal, Volume 20, 24 June 1717