I have a lot of criteria in my smart campaign. It would be useful for me to know when a filter I add doesn't impact the bottom line - instead of covering all bases, I would like a distinct view of each step of the filter. For instance, if I drop in a 'member of smart list' to my smart campaign, it would be nice to have a box with "5,302" show as my target list (or whatever the number may be). Then with each criterion that number will dwindle... If I add "in country = US" then it shows somewhere around 4,000, and so on and so on. The benefit here is if I add a filter and don't get any change in number - or get 0 - then I know a mistake has been made and can fix it, without having to come back when the project is done and try to figure out where the problem is.
It's not always useful when going to the Schedule tab to find what my range of target contacts is. Maybe there's a better way that I'm not familiar with? (This would apply to both Smart Campaigns, and Smart Lists, in theory)