This would be a nice feature to have in Fences. I work on a lot of various "projects" switching between them often. I generally have one area of my desktop where I like to store "stuff" that I'm currently working with. (If I just created a fence for each project, it'd take up the entire desktop easy and turn into a mess.) So I tend to do a lot of moving files around in and out of my staging/working fence when switching projects.
If that fence were simply tabbed (or perhaps had some sort of drop-down-combobox to select a fence), it'd definitely be less of a hassle.