This is a pretty big deal for designers and developers who love dual-display setups, e.g. for looking at code and docs at the same time. Until now you had to lug around a 15" MacBook Pro to get dual external monitor support.
Tangentally: One is inclined to wonder, if dual display support is such a big deal, why they all use hardware incapable of it.
For me, performance easily beats portability. I also have a laptop, but I'd never use it as a primary desktop.
If I need to go out of town, I just take the laptop with me and I can do everything I would normally do, just with a smaller screen.
At least for web development, speed usually isn't an issue ... putting in an SSD drive a year or so ago, though, was the best thing I did - very noticeable speed increase!
So, for me it's a convenience factor.