First of all — I'm speaking about adding this feature to OLD New dialogue window, because new fancy one is a disaster.

I create new documents with a vision in mind, what will I do and what grid would I use. I'd like to have an ability to set it with defining size of a document.

Next, most new users have no idea about grids and don't understand, why use it. More of it, even intermediate users don't have a clue, and I have to deal with a lot of designs, made by these average designers. All of them can't skip new document dialogue window, and if grid settings would be there, I would less deal with documents created in metric units with a grid set in inches and stuff designed by eye, all with gaps and holes, like attached picture shows. I'm idealist, I know, but still hope people will educate and do less bad design.