128GB is enough if you don't:
use bootcamp, or virtual machines/parallels/vmware
store a ton of media on your machine
play a lot of games (Starcraft 2, Portal...)
For an average user who does web surfing, Microsoft Office, keeps most of their music/movies on cloud storage for streaming, they'll be fine
But if you want to take your 50GB music collection, more than a handful of bluray rips, and that kind of stuff on the road, you'll want more space, or have to use an external drive. You can get 128GB USB drives for that stuff, or and external USB hard drive.
If you're going to do a lot of movie editing or working with tons of high-resolution photos, you'll start running out of room quick. And these days games can take 10-30GB each.
I used a 64GB Air for quite a long time! And 20GB of that was my music collection. 15GB for OS X, Xcode, a couple other tools, each project at work was around 1GB each, 3GB to mirror my Dropbox, I was pushing it quite a bit when it came to free space but it worked for me.