Why buy a Ninja Tabi?

Tenacity only matters if you get subjected to a tenacity-able CC effect. On teams which favor mostly knockups or suppressions as their hard CC, for instance, Tenacity is useless (as it doesn't reduce knockup or suppression duration).

This brings us to our second point: don't go into games with your heart set on a specific pair of boots. Adjust your build to best combat the team you're fighting against. Folks don't not buy merc treads because Tenacity is useless (it isn't), they buy Tabis instead because they are more useful against the opponent team.

You buy ninja tabi for a couple reasons:

1. you're laning against a full on ad champ
2. the enemy team doesn't have much in the way of ap champs
3. you're going to roll tenacity from another item (not boots)

If the other team has mostly AD, ninja tabi is really useful.

Also, if your laning against a hard AD, with no CC, ninja tabi could be for you.

Boots are a situational item. Every game you will have to choose which way to go.

The only boots that you don't really have to think about are Sorc or Berserker's because unless you're an AP or a AD carry that needs atk spd, you shouldn't be thinking about those boots. If you ARE, then you should look at those boots, respectively, and figure out if you should use those (more aggressive route) or be defensive and roll something like tabi or treads.

Also, ninja tabi is cheaper, to the tune of 400g, so keep that in mind as well.