This pretty much mirrors my reasons.
I move around a lot, meetings at remote sites, day trips, work for a few hours from some random coffee shop.
Also, I have my sideline job where I host a growing cache of customer servers in a datacenter I have some space carved out of.
Lastly, I spend most of my weekends at a second home and in the summer spend a sub-part of those weekends on my boat.
In all these cases a small, portable, light-weight machine is often all I need or want for the basic tasks I'll be doing. A netbook (Asus EEE 4G) with my Verizon USB EVDO dongle gives me portable computing, with or without Wifi available. The fact that it's pretty much impossible to store anything personal/private on it (especially if you don't store passwords in FF) and the low cost make it basically disposable if it gets lost/damaged/stolen/dropped in the lake.
If 99% of my job was spent in a cubicle I would probably have less use for it.
Sometimes it'll sit for a month, sometimes it will get used almost everyday.