I also choose to say that nothing is a weed; there are things I prefer and things I don't, that grow naturally. Many of these things have more to do with placement than existence. Things grow here such that making a marginal garden is more like taking a marble slab and chiseling away the parts I don't want, than like adding things I do want.
For instance, there is a lot of area with reddish heather with small pink flowers, that blends to a greenish tone and fades into moss or fern areas. This is fantastic and unusual ground cover; I love it but I need it in some different places. The trick is, maybe it just never made it over to grow in the areas that I want it in, but maybe it didn't grow there because the light is too dark/bright or the soil is a little wrong in those areas. I don't know. I can only try. There is some of that stuff growing a bit where I do want it; I think if I start to clear a space around it and give it some water and tend to it, it will spread. I'm thinking of digging some up and moving it where I want as well.
The marginal garden needs design and large areas of texture, using things that don't need a lot of tending, to be successful at being marginal. I also have to really like it there, or it doesn't qualify as a garden. So, I am going slow and learning what I have here, and to become conversant with the lay of the land.