It's extremely useful. In the case of very small aircraft, simply hand-hold the measuring unit to the control surface. There is no need for clamping. For larger aircraft, (for example, 25% - 46%) clamping to the control surface does not negatively impact accuracy at all. A slight manual "boost" (if needed) to remove any servo buzzing will always yield a precise measurement.

A weight of 8.5 ounces is not at all relevant for incidence, decalage and engine incidence.