I have a MBA, Windows 7 Desktop, iPhone 4, and a 1st gen iPad.
To me, each one has a unique purpose.
MBA - General device for when I need to type something more than a paragraph or lots of tabs open at once.
Desktop - Gaming, Software Development, and converting videos.
iPhone 4 - Always with me, great for quick google searches, apps, and time waster with games.
iPad - My usual couch/bed device for browsing web/reading books/email.
I don't feel like its overkill because each device services a distinct purpose for me. The only overlap, if there was one, would be the iPhone/iPad in my case. I tend to use the iPad for couch surfing and quick google searches. Obviously the iPhone is more important because it's my phone but for what I use it for most the time there is some overlap between those two devices. I build a new desktop around every 4 years and make one or two upgrades (not always) in that stretch of time. Usually a new GPU, more ram or something to keep it speedy. I am the type of person who is okay with Medium settings on my favorite games at native res. I do not need every single graphics option turned to max or else I would spend a lot of money.
To me, the iPad/MBA serve very different purposes and there is a lot that each devices does better than the other. The only real similarity I have between them is the portability. Both are extremely light and can be brought to places easily.