Best usage of menace?

Menace is a stat that I was largely unaware of prior to the most recent patch. Having the ability to reduce enemy defenses seems strong, but I have a handful of questions about how this mechanic works, and how to best use it in game.

1 - Does menace stack with attack speed, or does it have some sort of internal cooldown that prevents you from just dealing true damage at all times.

2 - How much defense does a unit usually have? Is it a base stat, or does it only come from items that give defense/guard? If the latter, does that mean menace is only valuable as a counter to your opponent's items? Why would that be good?

3 - Is menace best used on your main carry, on a subcarry/cc bot like haerang, or a frontline tank? This seems particularly relevant for the menace accessories and Weak Spot legacy.