The 2" of extra screen real-estate doesn't give me a huge benefit usually (not enough to put two windows side-by-side); but it does widen the laptop enough to make it harder to use in tight spaces (ie. airplanes), and it does make the laptop heavier.
On the other hand, at the time I bought my MBP, the 13" MacBook Pro was not a Pro. I'm honestly not sure what I would get these days. I might go for the 13", and get a bunch of external stuff (display, mouse, keyboard, but mostly the display).
I have a "seriously tore up" 15" MBP from spring 2008 which I carry with me for really mobile use, and a 17" 2010 MBP which sits in my "base" location for weeks at a time. I went that route, vs. the 15" HD, because I can't easily have an external monitor while being mobile -- otherwise, the 15" HD seems to be the best, coupled with an external 24, 27, or 30.
I am considering getting a 13" MBP or maybe an x301 or panasonic s9 whenever they next get bumped. I don't care for netbooks (too weak), but a long battery life small laptop would be a nice tool. The lack of a keyboard on the ipad cripples me; often, the blackberry is more useful.
I have a 13" aluminum MacBook with external display, keyboard, and mouse, and highly recommend that setup. I downsized from a 15" PowerBook to 13" MacBooks long ago, and with the aluminum ones ("Pro" or not) they are fantastic machines.
Because it's a fairly sturdy machine that makes a great portable workstation? For me it's because I'm invested in the platform: whenever I start a new big project I get myself a new one. (Bonus if I can convince someone else to get me one.) I like how there is absolutely zero friction in the purchase decision: I already know what I'll get & I don't have to go through the agonizing process of configuring the machine. (both hardware & software) It's an expendable high-quality tool that gets the job done.
Apple makes better hardware than most of the PC manufacturers. The only mon-Mac laptops I like as hardware are the Lenovo T, W, and X series. I have some IBM/lenovo laptops too. Some of the Panasonic business-rugged laptops (W, T, S series) are also nice, but driver support is often weak.
Linux itself on laptops sucks more than on any other platform -- driver support, ACPI/APM support, etc. all suffer vs. Mac. OSX is a pretty decent Unix OS (although since I use ubuntu for most of my servers, the slight syntax variation vs. linux gets annoying). I also really like having Keynote, Pages, and Numbers available -- OpenOffice just doesn't do it for me. And, I use Adobe CS4 (mainly Illustrator and Photoshop). And for a gui text editor, TextMate is nice. And OmniGraffle.
Basically, Mac commercial software is a big plus, but fundamentally it's about having a great working Unix laptop which requires minimal tweaking out of the box.
