The NEC voltage drop (5% at receptacle) is a recommendation for efficiency, not a code requirement. So wording needs to take that into consideration, but high VD at a receptacle can be an indicator of hazardous issues with a circuit.
The HI cannot tell what defects might be in the circuit, such as how the circuit is wired: series (back-stabbed or “speed wired”) or parallel (pigtailed), circuit run too long for AWG used, has high resistance connections (loose connections at pig tails?), worn or corroded receptacles (tester easily falls out or does not achieve a solid connection), etc. AFCIs can go a long way toward protecting circuits, but can fail over time – especially if not tested monthly (who does that?)
The INSP-3, properly used, can tell a lot if an electrician is willing to do the work and someone is willing to pay for it to be done. According to Test Equipment Depot.com, the SureTest 61-165 has been discontinued. Perhaps we can expect an “updated” version of it?
If VD seems excessive (in excess of 10%, as some municipalities say), its probably best to list it as an indicator to be verified by a licensed and bonded electrical contractor. That said of course there are some mighty fine electrical contractors (who will investigate) and perhaps some not so fine (who will poo-poo the recommendation). But that’s the same in all professions – good and not so good. I’ve had an electrician tell the client that a three prong, three wire dryer receptacle is the latest standard and not four prong, four wire (2008 NEC). Yes, three prong can be “grandfathered in”, but four prong is still safer and can be recommended as a safety upgrade from a three prong. But that’s all we can do is to mention it and defer it, but not make the “final” decision.
I think its important, if one is to use such a device as an INSP-3, to mention the condition as an indicator to be further evaluated. Should a HI use one? That’s a personal decision. Does it’s use increase “liability”? That’s a legal question I’ll bet will get different opinions, depending on who wants what from whom.
Yes, its not in SOP. Should it be? Who knows? Am I going to use it on every inspection, probably not. But only if there seem to be some potential issues such as dimming lights under load, an older house that was originally wired with two prong receptacles, a mixture of two prong and three prong receptacles, etc., I might pull the INSP-3 out and test. But that will be on a case-by-case basis. I don’t want to send an electrician on a goose chase, only to have him/her tell the agent, “the home inspector doesn’t know what he’s talking about.”
So who knows. The HI needs to decide for him/her self. It seems to be quite controversial to say the least, whether or not a HI should test for voltage drop.