The power armlet is light hand armor with a strength boost. Whether you use it is largely based on what ability boosts you have covered by other slots. It's a good piece of armor in terms of defense vs. weight, and resists poison just for good measure.
The Power Wristband is an excellent accessory in Chapter 2. Not only does it give a significant boost to the wearer's Strength score, it will grant the Physical Wall: 5 ability to anyone in the party except Sazh or Hope due to the available weapons. This ability can be a big boost early in the game, especially against enemies with multi-hit attacks.