Partial Product Imports - Update only columns included in import doc

I've been loving the Product Imports/Exports improvement! However, there are limitations that have hindered us from using it more frequently - no attribute columns, 5 user group columns, no secondary images. (I get it - including all the extensive product options in one import document is impossible.)

What I am asking for is a way to continue to use the Import/Export feature after you've set all of those missing items manually (otherwise, they get overriden with each new import).

Having the import only update the columns included in your import doc would hopefully allow you to keep those attributes, secondary images and 7 user group assignments, and only update the things you're intending to change - like a part number or product name.