I have a smartphone, that is always within a few feet of me (usually in my pocket, but sometimes laying on whichever desk I'm sitting at). I use it for calls, SMS, email, and rarely internet browsing or gaming.
My laptop is always nearby, too. It is a ThinkPad X200 (light weight, C2D P8600 2.4GHz, 12.1" WXGA, 8 GB RAM, 120 GB SSD). It's my primary computer at home, work, and everywhere else. It saves me a tremendous amount of time to be able to use one machine for everything... I don't have to adjust to different configurations, or having some software installed on certain machines, and not on others. The 12" LCD wouldn't be sufficient for full-time use for everything, so I have a docking station at work, attached to a 23" wide LCD. The smaller laptop LCD is usually acceptable for home use, but I'm planning on getting a docking station and external monitor for there also. I carry the laptop with me most places (home/work/hotels/etc). I can't count the number of times it has been beneficial for me to have it with me, while I'm in the middle of nowhere, and need to quickly log into a remote server and do something. It's awesome.
<edit> I hadn't thought to mention it until I read IronWing's post, but I also have a ton of GIS tools on my laptop, that I might need to use in event of an emergency. That, in and of itself, justifies carrying it everywhere, IMO. </edit>
There is still a desktop at home, and at work. My home PC most of the time acts more as a file server than anything... though I occasionally sit down at my desk and use it (when I want to watch videos on a larger monitor, just feel like a change of venue, etc). The desktop at work I use mostly as an RDP server, so that I can keep certain programs running, and ready to be used at a moment's notice.
Yes. Laptops are very useful.