My thing is that at times I have projects I need to leave running, or that are going to be CPU intensive for some period of time yet I still need to be portable.
So having a setup like above, with a standard DVI based monitor so a two port KVM switch can be used, I can choose between dual monitor for my laptop (my primary, day to day system), or the internal screen on it, and the Mac Mini on the monitor. All with the option of leaving the Mac Mini to do whatever, download that group of 3.4 gig DVD ISO images I need, or process through a huge file set, or whatever the case may be, while still being able to run out the door at a moments notice with my laptop in hand.
There are a number of things to be said about having a backup system too in the event something happens to your laptop (stolen/lost on trip, in for repair, tied up for that presentation in the other room for another hour, whatever). 😊