In situations where you aren’t pushing the amp hard, say, when the master volume is turned way down low, to bedroom volume levels, you aren’t approaching the limits of the amp’s headroom, so you may not notice much of a change in the volume at all in this situation.
It may also depend on the sound you are using. For example, we fired up an Orange TH30 combo, and set the volumes for the clean and dirty channels as low as we possibly could for each channel — the knobs were just barely cracked open. In this situation, there was no noticeable volume difference when changing from full to half power on the clean channel. However, on the dirty channel, there was a difference, although it was almost more of a tonal difference than a volume difference.
At those settings, with the half-power switch, you’re really affecting headroom more than you are output. As you turn up the amp, the affect of the half-power switch makes more of an impact on the output volume.