Full Boots on yas or shockingly good (I will say teir 1 boots are kind of mandatory IMO though..but not teir 2 boots), I will admit, but mostly early game and good for hit and run tactics. If the enemy you lane against is sticky ..then its a total waste. For example, laning VS irelia is the worst if you go for boots..because she will just dash to you and stun you if you try to hit and run. First and foremost, I will say ...zerk greaves are NOT ideal when youre fightning more than 1 tank/bruisers that are good champions right now (such as Jax or Irelia for example) ...this is because you will most likely be going BORK, meaning youre now going to have ZerkGreaves, PD and BORK ..totally surpassing the 1.33 AS CD cap on his q ...and will be taking in a crap load of damage from those bruisers too without the Tabi or Merc.
When you get to 6 most of the teamfights involve brute damage (as you have your ult to stick to guys, and your team fights start happening where CC comes into play); where every time youre fighting someone and movement isnt part of the fight (as in youre standing there whacking them), you're theoretically losing power because the gold was spent on movement which is not being used. This is why a yasuo that built PD and 1 agility cloak will do far more damage than the yasuo with zerk greaves + zeal + 1 daggers or whatever ... but its all situational in the end anyways ..I mean what if the teamfight went well but then the guy flashes away and now you cant catch him because you didnt buy boots --- and you lost out on that KO? In the end man ..I dont think anything really matters other than how well you play with the items you shoot for.