> as this board is not a commercial product but rather
> a form that allows evaluation of the P5020, maybe they
> felt the need to include this option.

Yes, makes sense to give the evaluator the option to evaluate the different SerDes configs as well, even if he most definitely won't want to give his own customers that option on his own commercial board later on.

> I was more surprised by the lack of an X4 configured
> PCIe slot in the default setup.

I bet the P5020DS's main target audience is not developers of boards aimed at desktop computing ;-)