Edge is an attack/magic/support character, so if you expect to throw him in the front row and be a power character like Cecil or Kain, then you're going to be disappointed. With spells like pin, he can stun enemies, or with image he can completely negate damage, which at that point is the time you can put him in the front row. The speed that Edge has compared to Cecil and Kain allow him to take more turns, so the amount of damage that he does will be somewhat comparable.
When you get Edge, you'll want to put him in the back row with the most powerful long range weapon you can acquire. You'll take off a bit less damage than you would with melee weapons in the front row, but your defense will be doubled. So keep Cecil and Kain in the front row, Edge, Rydia, and Rosa in the back row. Once you get to the end of the game, keep Edge with the best long range weapon and Muramasa(or Masamune, I forget which is stronger,) and leave him in the back row.
*edit*
If you ever play Final Fantasy 4 The After Years, which the 10th chapter is amazing and is what makes the game, Edge is the best DPS melee character in the game by far. Deals some of the highest damage per hit and is way faster than every other character in the game. His only weakness is lower HP and defense than the heavier characters.
