The 4.5 pound 13 inch MBP is not in the MBA's category because it is 50 percent heavier than the 3 pound MBA. Even carrying a 6.6 pound 17 inch MBP isn't going to hurt anybody. That's why this "go to the gym" talk has been so exasperating to me and some others. The appeal of the MBA isn't the avoidance of physical injury. Instead, it's a matter of mobility and ease of use in mobile situations. It's in a class by itself where that is concerned. That said, the 13 inch MBP is a vastly more practical computer than the MBA, for a lot of reasons. That's why I won't be buying an MBA until and unless Apple redesigns it in a way that provides a lot more power and flexibility than it has now.
The MBA is more than practical for the AVERAGE consumer. If one were to try to edit audio or video, then, yes, the MBP is a better choice (actually, an iMac or Mac Pro is the best choice); but for email, surfing, word processing (the daily tasks of a computer user) the MBA is more than capable.