For architectural or landscape work the mitutoyo is an essential piece of gear for me. I'm oftentimes swinging or tilting the camera and using f-stops that don't allow much wiggle room for user error. Getting your standards aligned within a tenth of a degree makes for critical sharpness across the frame.
I recently used this tool to verify a substandard CLA on my Sinar P2. Turned out the Sinar-trained tech did not set the zero detents properly resulting in a 1.5 degree deviation from parallel - enough to throw a horizon line from a 5x7 out of focus at f16. When I confronted him about this he had a difficult time believing there was a problem and an even more difficult time believing I could measure the exact amount of deviation between the standards. So I shot a two-minute video demonstrating the deviation by using the mitutoyo digital protractor. That changed his tune pretty quick.
The Mitutoyo is about the same length and depth as a leica M and approx. 2-1/2" tall. Similar build quality as well. Also nice is the ability to manually recalibrate and set the zero point.