I subscribe to the "different horses for different courses" theory, and as such, I float between gargantuan, massive, large, medium, compact, small, and miniscule rigs in an effort to bring the "ideal" rig to a given gig. At least that's one reason which I use to attempt to justify my gear lust.
The advantages of bringing the "right" gig to a rig are several. The most obvious one is the simple fact of fitting the rig to the size of the stage area (and to a lesser extent, the venue as a whole - although if there is no PA support, then this actually becomes the most important factor). Another is the feeling on inner piece knowing that you have brought enough rig to handle the task at hand, but that you have not forced your back to carry "unnecessary" weight by bringing too much rig (this is one that I struggle with...

). In addition to "how much" rig to bring, I find that when I am working a room that I know well, being able to bring a rig that naturally compensates for the rooms anomalies is helpful (but really, a parametric EQ or two can handle this, too).
Okay, enough rambling. Some examples of the bigger rigs that I have brought would include:
and
A slightly smaller rig:
The smallest rig that I would consider playing a loudish rock gig (with PA support) is a Walter Woods Ultra into a Hevos Midget. Yes, a 1x10, but it is a really, really loud 1x10. For acoustic only jams, the Phil Jones Briefcase gets the nod.
One rig which really hits a sweet spot is a Walter Woods Ultra into two EA VL-208's. I brought this mighty mini rig to a recent benefit, and I was killing other rigs using much larger cabs (there was an 8x10, a guy using two 4x10's, and a 4x10/1x15 setup). The sound guy kinda laughed at it as I was setting up, but then had to come ask me to turn down because I was overpowering the PA!
Later, Tom.