I've long been an advocate of making things as light as possible, but no lighter. If you have two options that do the same job equally well, but one is a pound lighter, there's no reason to go with the heavier option.

That's a really clever idea for mounting. Panniers do tend to be rather heavy, and this makes a lot more sense (and a lot cheaper) than your other option.

I've actually pondered a similar concept, welding a rack specifically designed to hold light weight stuff sacks, but it's low on the project list and ease of access would be an issue.