How does the item mastery system work in Infinity Blade?

I must have dozed off when the game was explaining it to me, but I have no idea what the point of mastering weapons is.

As such, I can't figure out if I should be using a weaker weapon till I master it, then selling it, etc.

When you defeat an enemy, you are awarded a fixed amount of experience for each unmastered item you had equipped in the fight, and for each item, that much XP goes toward moving your character to the next level. The total XP is essentially a multiple of your unmastered items. So if an enemy is "worth" 500 XP, you'd get 2,000 total XP if you are using an unmastered weapon, shield, armor, and helm.

If an item is Mastered, you no longer gain experience for using it. So, if one of the items you use in a fight is mastered, then the portion of the XP earned for that item doesn't go toward moving your character to the next level - it is effectively lost.

IMO, the only time you'd want to use a mastered item over an unmastered item is if you are having a hard time with a fight, and the mastered item is stronger than the unmastered item.