I own a Pro and an iPad. Used a MBA plenty of times.
The Pro is a converged device, the first of its kind, disrupting a bunch of different markets. You got a device that acts like an iPad, a MBA, and a Mac Mini all rolled into one. It's like when the first smartphones came out and disrupted phones, PDA's, cameras, etc. Notice the first smartphones also didn't do these tasks as well as the devices they disrupted. Yet they're everywhere now.
It's not gonna kill either market any time soon but it's a welcome paradigm shift that could become a gold standard in a few years, especially because Intel's roadmap is set to make midrange x86 chips extremely viable on mobile and because MS is currently laying the groundwork in creating a singular mobile/desktop OS.
There are a lot of tablet benefits to using the Surface Pro. For one, it uses Win 8 instead your typical mobile OS. With a full desktop OS, you don't have to wait for things like ARM based apps for web services (IE Yelp, Tumblr, etc) to hit app stores - you can just pin the actual website and get full functionality. You also have a blazing fast SSD instead of slow flash memory. You can immediately tell the difference doing everything from browsing the net to using it as an E-reader.
Far as an Win8 Atom tablet vs the Pro, you wanna play a game or use something that requires processing power, you're gonna need that i5. I'm running Sim City and Guild Wars 2 on my Pro. Those games would slideshow on an Atom device.
Something that's not talked about a lot is you can also use it as a small form factor PC like a Mac Mini. When I bring it home, I stick it on my desk and hook it up to an HDMI monitor and the touchscreen basically becomes a huge trackpad.
All I use the iPad for nowadays is clicking on mines in Clash of Clans once a week.