Rinoa, despite her appearance, actually makes a sturdy addition to your party. One perceptive player on a forum I was looking over brought up the fact that she has just as many hit-points per level (without junctioning) as the hardy Squall. I never really used Rinoa much, but properly junctioned she makes a fine fighter and a great sorceress- I mean mage! Doh!
Rinoa's limit break is different by any FF's standards. She summons the mighty GF Angelo! Angelo is a mighty beast on four legs covered with fur. His long snout can rip any enemy to shreds. He also can sit up, roll over and beg for doggy treats. Yes, Angelo is a dog. A powerful dog, though. You can teach it tricks that it uses to pummel the unsuspecting enemy. Rinoa is the only character in the game to have 2 limit breaks. Her second, which you get later on in the game, makes her go berserk while casting insanely powerful magic at no cost. Pretty nifty.