Good question! Given I work for Unified Inbox, my reply is more about the nature of a unified inbox that what Sparrow are doing.
So the reason I use a unified inbox is that I only have to login to one place. I can then choose (using the filters) what work I'm going to concentrate on - eg I might look at the support@ addresses, or my personal ones, or all the addresses of one company/project. It can take a while to learn the use the filters well so you're not presented with an overwhelming list of everything but I'd hate to go back to individual inboxes now :)
So the reason I use a unified inbox is that I only have to login to one place. I can then choose (using the filters) what work I'm going to concentrate on - eg I might look at the support@ addresses, or my personal ones, or all the addresses of one company/project. It can take a while to learn the use the filters well so you're not presented with an overwhelming list of everything but I'd hate to go back to individual inboxes now :)