Well, the motherboard manufacturer isn't always the computer manufacturer. Typically I'm helping individuals with OEM computers (Dell, HP, Toshiba, Asus, etc.), and I need to find the exact brand and model to point to the right drivers to install. Searching by motherboard is a waste of time, and asking the owner rarely works.
I come across custom builds occasionally, but don't usually deal with them (somebody else will always know more than I do about customs).
It seems like it would be a simple thing to just add the brand and model in the summary page. It would make my job a lot easier.
Hopefully this will be implemented in a new release soon.