Faster is not always better , eyeballing it is faster. Why would a hammer be faster to measure with?
Measure with a screw driver it would still take the same amount of time.The item you use to measure has nothing to do with the speed of the measurement.
What I look at is consistant box hieght with a hammer you must move your measuring device (hammer) that allows for the real possibility of the box moving creating inconsistant height.
With a 2x4 it stays in place during the nailing process thereby giving a greater probability of a consistant height.
Using a hammer means only having to handle two things.... the hammer and the box. Using anything else to measure with, no matter if it's a tape measure, a 2x4, a stick of conduit, a screwdriver, or whatever else you want to use means you have to juggle three things with only two hands.
If your hammer is a little crooked from one box to the next, it may be off 1/16th of a inch......... so what? Is anyone gonna notice that? No.
If you're concerned about the box moving, just hold on tighter. It's called work for a reason.