Laptops are great! There are small powerful gadgets that you get to drag to anywhere you want and use them as they come. I love laptops! Laptop is like your super-sized personal organizer. I use my laptop for all my office-type-of-work. By that I mean e-mailing, writing documents, blogging, Internet whatevering (substitute with browsing, banking, surfing, etc.)
They are great, if you are using them by yourself. But when it comes to pairing, they are no good. And I tell you why.
Laptops have usually smaller screens.
The navigator can not see as clearly what the driver sees (if anything at all, depending on the screen resolution).
The control is harder to pass around.
When the control is passed to the other person, that usually means that the whole laptop is handed over.
Laptops are less powerful then full-blown desktop PCs
One could argue that it is not true, that laptops can be as powerful as desktops. Yes but at what price? And all we developers need is computing power to run the compilations as fast as we can.
Laptops are usually more expensive
You can get more on desktop for the same price. No need to say more.
On a bright side, the laptops are tax deductible, and you can salary-sacrifice it in one year. Every year. Well, in Aussieland we can. It takes three years for ordinary PCs to be written off.
Laptops are harder to upgrade
They are simply less pluggable than desktop PCs. Therefore they age quicker.
Would I choose to have a laptop? Definitely! But not for pair programming. For that, I'd choose to have a nice and fast desktop PC.