I'm with Step Turn on this one.
Firstly, different types of engine and engine control/operation here.
Rotax is simple - reduce rpm to idle, off with one ignition switch and rpm decays, off with the other and engine stops. Thus proving the switch is doing its thing.
Lycoming or other (non-Rotax) aero engines that have two independent ignition switches obviously behave like the Rotax.
Lycoming or other (non-Rotax) aero engines with a keyed ignition switch should be treated differently.
For sure, checking the 'left' and 'right' positions will tell one if in that position the switch is earthing out (switching off) one mag or the other.
The 'off' position may have an internal problem of its own therefore allowing one or both mags to be live. Hence, the 'dead cut' check is carried out - but only at idle where it won't cause a problem when the ignition is turned back on.
If someone goes to off accidentally when checking the mags at any rpm other than idle, then you're better off letting the engine stop completely and then restart it.
Personally, when checking mag timing on maintenance checks, I always try moving the key about in the ignition switch in all possible positions (including off) with the mag timing box 'on' to make sure the switch is behaving as it should. I've replaced a number of ignition switches over the years because of an intermittent live mag when the switch was in the 'off' position. I also check to make sure the key can't be removed in any position apart from when 'off' - again, I've found worn keys or switches that fail this check.