The point of the Iorveth path is that by playing it, you get a lot of key pieces to fill in the puzzle that is the Witcher 2's story. If you side with Iorveth from the beginning, you learn:
- That Philippa Eilheart really is a villain, regardless of her goals or motives.
- All about why there was a dragon at La Valette castle and what the story is with that.
- That Triss is essentially a well-meaning idiot rather than a villain, and what happened to her.
I actually think the Roche path was more enjoyable, and it's clearly presented as the 'official' choice, but it's almost impossible to piece together who several of the key characters in the game are (and what they're really up to) if you go that route.