We are implementing CPM here within APJ. My client really love some of the CPM reference AO workbooks and want to use them productively. But my BW consultant recommends that we copy them to use them and it's minimal effort. [As SAP documentation says they are reference workbooks] What's the best practice? Any downsides?
Here is a blog on how to build a custom workbook ground up. if you do not have enhancements to planning model, this approach is the quicker and clearer.SAP CPM, creating new planning application using planning content
If you find CPM reference workbooks to be best fir for your scenario, you can very well continue use the same workbooks. In case you want to edit the workbook, I would recommend you to copy the workbook. As you said the only effort involved here is just copying and saving the workbook apart from the changes you require.
This way you will always have the standard or reference workbook for quick reference and your edited one with extra fields or queries for productive usage.