You can write class name without import statements, when editor shows you that this reference is unresolved, you can just press Alt+Enter, and plugin insert essential import statement automatically.
Options for this feature: File -> Settings -> Code Style -> Scala -> Imports
Scala language gives you flexible ways to use import statements. You can import not full qualified name or use import statemnt in any code block. Also you can collect few imports in one import statemet without using placeholder. All of these Scala language features we tried to reflect in plugin:
This can produce bugs in plugin. With using placeholder, you automatically import packages. So inserted import can be hided, it can be rare case, but must be fixed.