So 
increases your movement speed when out of combat, and hecarim gets more damage with more movement speed. I read on the LOL wiki, that his passive updates every 0.25 seconds. So technically if you get a first hit with hecarim, he will still have the bonus AD for 0.25 seconds and will do more damage. So are boots of mobility worth buying for ganking damage?
Droogzy1 votesIn short no.
You're better off with Merc treads or Swifts (in that order) on Hecarim depending upon how much CC other team possesses. The extra damage isn't really worth the trade off of a shot at the extra damage when Mercs can provide a consistent level of reduction from CC or swifts can provide a consistent level of damage increase while providing slow reduction.
DoktorKaiser1 votesMobi boots are trash on most champs, I personally buy them in maybe 1% of my games, especially on Hecarim who needs high movement speed constantly for both damage and sticking to target so he can spam q.
