Despite me being a Cortex dealer, I would say that 99% of jet's don't need a gyro. There are the odd ones that do but they are in the minority.

However, there are reasons why you might want to install one. It may improve the characteristics of your particular model (wing rock/fishtailing etc), it may make landing and take-off more solid (making it easier), it may make you more confident to fly in stronger, gustier winds, it may be that you like the technology and want to explore its possibilities.

In essence, in most cases there is no need for one, but if you want one then go for it.