Well, I'm not sure I understand pure spotting - I can never figure standing under the flight path without a camera
But when it comes to photography ... well I guess for me aircraft are the perfect example of form following function - these things are, in my opinion, the epitome of design. Couple that with their natural environment - I mean the sky and clouds can be just beautiful, but most people just give it a passing glance - add a flash of sun off polished metal, some wild colour schemes - photo heaven! Plus there's the thrill of the chase - both in capturing that rare bird, but also looking for a way to portray a 737 in a new way.
For me the "spotting" is an incidental - I don't collect regs, but like any specialised photography, the more you know and understand your subject, the better able you'll be to get the right results.
I think anyone who has seen people hanging around the airport taking pics and decided to have a go themselves has discovered there's a bit more to the hobby than would appear at first glance - esp. when they try to upload to A.net!
Colin K. Work, Pixstel