People are pretty harsh here considering it's a forum about student deciding which of two incredibly well-designed laptops they are buying.... There are plenty of 90lb college students and the 1.5lb difference between a MBA and MBP is the difference between carrying 3% & 5% of bodyweight - not trivial.
I would advise most people to put up with the MBP's extra weight though because the MBA's battery life would be a pain for a student that might face multiple classes with no outlets in a day. Also with the MBA you would have to carry around the power adapter, which might not be necessary with the MBP. Or one could certainly buy a 21" iMac for home/dorm and a netbook for taking notes in class. Cost would be about the same as a MBA.
I think for many of us, myself included, it is difficult to imagine what it would be like to be 90 lbs so it is much easier for us to think "1.5 lbs? are you joking? that's nothing!" when in fact, it really is a substantial percentage for the particular individual. I agree with your points though that you gain so much from that 1.5 lbs it is worth it, especially if this is an all purpose machine (i.e. you don't have a dedicated home machine and a portable).
And there are probably plenty of 250+ pound businessmen who would rather not carry around a 4.5lb laptop when their needs are met by a 3lb laptop. It's really a matter of personal opinion/preference when one is deciding how heavy is too heavy. It doesn't matter how much someone weighs or what their bodies are physically capable of carrying, whether it's a laptop or books or a laptop + books.
I'm 28, 5'10, 190. I have a 3-year-old boy who loves to be carried. I'm not in the greatest shape, but I'm fairly fit. Personally, I think I could sacrifice some performance for the weight savings because when I do travel, I'd rather be light.
I ended up being a little sarcastic because it irks me when people don't understand that weight is an important consideration for some people, regardless of performance. Perceived differences can be just as important in comparison, regardless of the actual numbers. (Additionally, I've come to realize that some actual numbers are pretty meaningless).