Imagine Revit prompts you on the names of existing Family Parameters inside your family while you are typing your fancy formula. The same or similar way as advanced text editors or programming IDEs work - see Notepad++, SharpDevelop or Visual Studio for reference.
If you want to use an existing parameter name you could select it using arrows and TAB while you type.
That would speed up content development and make it a lot easier! And less error pop-ups.