For people who want to run one gun only and switch from day to night, they are a nice option. They are very different than NV clips as NV allows seeing through glass. Think of a thermal clip on as watching TV with a spotting scope because this is what is really happening. The reasons I don't use them are they are more expensive, the focus is in front of your day-scope, and you are adding more weight with two optical devices on one gun. On top of the fact thermal clip ons are more expensive than their stand-alone counterparts, they tend to work best with LPVO style scopes which are typically expensive as well. There are some diehard clip-on fans but I am not one of them.
For those that like to record your hunts, there is no reticle because the internal recording of the thermal is happening in front of your reticle.