Okay, I see where you are coming from. I do use G’MIC as a plugin but I started using the CLI for these reasons:
1. Preview is not representative of the final outcome.
2. The GUI really isn’t there yet. Other than a few “tools”, the plugin is merely a series of forms that gather input for the underlying code of the filters. Stuff like tooltips simply don’t exist. However, the filter coder could add a short description of the filter and instructions if he or she wished. I have hinted at wanting certain features but only in passing. I would say that it is not a priority on people’s mind. 
That said, the plugin works well with selections and masking in a way that is more difficult to achieve in CLI, so the plugin is not without its advantages.
3. Most filters are not adapted to float and other cases that I find myself working in. In CLI and building my own filters, it is easier because I have more control on what I want to achieve. And it is fun to experiment!