Short and sweet : if what you are doing is easy it has stopped being effective!
The goal of training should be create an adaptative response. If you workouts are too easy there will be no adaptative response and you are wasting your time.
Your training should be challenging enough that your body HAS to adapt to the stressor and be ready for the next session. This does not mean that you have to crush yourself every time you train. That's ridiculous and leads to diminishing returns... but is a common practice at a lot of gyms and endorsed by a lot of trainers!
So how hard should your training be? It should be hard enough to create the requirement for an adaptative response AND not so hard that you can't recover from it before your next training session.
But... EASY has no place in your training... EVER!