The "Use" ability is hands-down one of the best abilities in the game. Just stealing from regular enemies gets you such good items that outclass much of the other party members' abilities. You can steal Poison Fangs from any wasp-type enemy, and they deal damage and inflict poison with 100% success rate - completely outclassing Lulu's Bio spell.

Not to mention Smoke Bombs, Sleep/Dream Powders and Silence Grenades, inflicting Blind/Sleep/Silence with 100% success rate for THREE turns on the ENTIRE enemy party, again completely outclassing ALL of Wakka's abilities except Triple Foul. And you can steal them from any bird-type enemy (Smoke Bombs), wolf-type enemy (Sleep/Dream Powders) or mushroom-type enemy (Silence Grenades). Not to mention Frag Grenades (rare steal from some machinas) deal Armor Break. And from the soldiers on the Highbridge you can steal Purifying Salts which dispels an enemy and deals damage at the same time, again outclassing the simple Dispel spell. And I didn't even mention Mix.

Yep, Rikku can do some really stupid trout.