I have used one in the past; however, I was inconsistent about keeping it damp. My violin shop owner (also a cellist) told me that for my climate in Oklahoma it did not matter if I used one or not. The key is to use it properly (damp) all the time or don’t use it al all. As I seem to forget to keep it damp, I removed it. Perhaps if one live in a very dry climate such as Arizona it may be more necessary. In OK we have quite a bit of humidity.