All abilities on weapons & armour are "mastered", but only when they are equipped.

Eg:

Tidus:
Weapon:Razzmatazz
Armour: Peaceful shield

Weapon ability: Slowtouch, Darktouch
Armour Ability: No Encounters

The weapon abilities will induce slow and dark status effects to enemies, but say you equip the Hrunting, with SOS Overdrive, you will stop inflicting slow and dark, and instead your overdrive gauge will increase faster at low HP. This makes it more interesting when choosing epuipment for battles.

The same applies to armour. With no encounters, you will have free rein of the area, but equip something else, and you can expect to fight battles.