Here's one example we just faced where this would have been useful. In my situation, we created a Segment that was to include all recipients in three different Contact Lists. When we created the list, we erroneously used AND, and therefore created an intersection of the three lists, or only the 69 emails that were common to all three (out of a total of 1,653 unique recipients in the three lists). We didn’t notice this until after we had sent the email. I thought I’d just download a CSV of the 69 unique recipients that did receive the email, and then upload it as a new list to exclude in a new segment to email the remaining recipients. I discovered at that point that what I wanted to do, i.e.,

New Segment Rule =
OR Contact List IS “list one”
OR Contact List IS “list two”
OR Contact List IS “list three”
AND Contact List IS NOT “list with people from above who have already received the email”

was not possible.

This would (supposedly) create a combined list of the first three Contact Lists, and exclude only those that received the email the first time. However, you can’t currently combine AND and OR in a Rule; it is either one or the other (probably because it is more complex to program the logic and precedence of the operands).

What I’ve done, as a workaround, is to download the entire combined list, and go through and delete by hand those 69 that received it the first time, and uploaded it as a new list to use in the campaign for a second mailing to reach the whole group.

I would find this VERY useful. This feature really needs to be added.