Medium to large companies, they need some sort of calendaring and meeting room booking system. This is where outlook (more importantly exchange) comes into the picture. I don't think it's the best solution out there, but it's a solution that works reasonably well.
I also don't really like that email and calendaring are tied together, I'd love to see a robust solution that works across Windows, Mac, Linux, Android and iOS. Yishan Wong wrote about this a few years ago.[1]
So, this would work great for personal email and small companies, but I don't see it replacing the email client of the larger ones. I'd love to see gmail and microsoft give some of these search/indexing features.
[1] http://algeri-wong.com/yishan/great-unsolved-problems-in-com...