My impression was that the other signs are only useful in the higher difficulty settings. I used Igni at the beginning of the game(normal difficulty) but later on, during some of the more challenging encounters of the game I had to use Quen. Alchemy on the the other hand was never used unless the quest forced me to do so. I remember using Cat Eye a lot more in The Witcher 1.
I still haven't played 2 which is stupid...just 1 and 3.