Do I need to buy SketchUpPro for use with OpenStudio?

edit

No you do not need SketchUpPro to use with OpenStudio. The free version of SketchUp works with it just fine.

It's more of a legal issue as to whether or not you need to purchase SketchUp Pro. The license agreement for SketchUp Make says...

Trimble Navigation Limited and/or its affiliates ("Trimble") gives you a personal, worldwide, royalty-free, non-assignable and non-exclusive license to use the executable version of the Software for non-commercial use only. Non-commercial use means: you may not sell, rent, lease or lend the output of the Software or the Services. If you are a for-profit organization of any kind, or an employee of a for-profit organization using the Software or Services in that capacity, you are engaged in commercial activity; therefore, in order to use the Software and Services, you must purchase a SketchUp Pro license.

So if you're creating energy models in OpenStudio with the aid of SketchUp in any capacity for-profit, you need to purchase SketchUp Pro to not be in violation of the SketchUp Make license agreement. At least that's how I read it.

Importing DWGs is a feature that is only available in SketchUp Pro. This can be a differentiator depending on your workflow to create geometry.

You do not need to buy it, the default download is SketchUp Pro and as you said it gets downgraded to SketchUp after 1-month wich is enough to use with OpenStudio, for non-profit purposes.