I just sold my 2011 MBA with a 256gb SSD and got a 2013 MBA with a 512gb SSD. If I could have gotten 512 back in 2011, I definitely would have. Yes, it's expensive but a year from now I really doubt that you'll say "damn, my SSD is just too big".
I like to keep as much stuff as possible on my MBA, and I didn't have a big problem with 256 in general, but I had to regularly prune old Logic Pro files to keep them from getting out of hand. And I would have liked a larger iTunes library.
The final straw for me was a video project that I recently did, after a few years hiatus. Within a couple days, my free space went from 110gb to less than 50gb and I had to keep deleting or archiving files to keep it at that level. Sure, you can use an external drive but that destroys the whole beauty of the MBA - the portability and fast SSD.
Unless the economics change, an aftermarket SSD upgrade is not the most cost-effective solution. I kept thinking about that for my 2011 machine, but they aren't cheap and it made more sense to just get a new machine with 512 and take advantage of the faster processor, better battery life, USB 3 and other improvements.