Talk:Mercury's Treads/@comment-91.181.209.32-20120108113927/@comment-24.113.84.116-20120109002012

isn't used much because it's not an endgame item. At the end of the game, everyone needs a pair of boots. However, for an AD carry, the attack speed and critical strike chance isn't as potent as some other item such as. So, since you are guaranteed to have boots until the end of the game, putting an important stat such as tenacity (in addition to the magic resist) on boots is an effective strategy.

So, taking your example of Cloak and Dagger, let's compare:
 * Cloak and Dagger + Ninja Tabi: +20% attack speed, +20% critical strike chance, +12% dodge, +25 armor, Enhanced Movement 2.
 * Phantom Dancer + Mercury's Treads: +55% attack speed, +30% critical strike chance, +15% movement speed, +25 magic resistance, +35 tenacity, Enhanced Movement 2.

Yes, the second set is more expensive, but in the endgame the effectiveness per item slot is what matters, not effectiveness per gold. You could try the first set, then sell them later when you have the money to get the second set, but most players generally just go with Mercury's Treads to begin with.

Hope this answers your question.