Is the use of defense mechanisms healthy?

With a few exceptions, no. Generally speaking, there are much healthier coping mechanisms available, and the use of say, denial, displacement, or projection would be considered less than ideal (or even pathological in some cases). We all utilize defense mechanisms from time to time, but relying on them typically makes things worse, especially in the long-term because as a rule they distort reality and prevent us from effectivlely, actively dealing with a problem. Some defense mechnisms, like sublimation, could be considered healthy, though.