Faust

Certainly Objectivism does not hold plumbers, electricians, builders to be "moochers," let alone "useless."
Honestly, when we speak of "utility" and one's "role in society," it makes me a touch uneasy. An individual is not obligated to have a "useful role in society" as judged by me, Ayn Rand, or anybody else.
Worth less to whom?
Every individual is (or ought to be) supremely important in his individual endeavors to himself--that's what matters. It's the only thing that matters. To be honest, I don't understand the point of Nicky comparing "electrician A" to "electrician B," either. You're right in your reply that the truth of the matter depends on their individual context. So much does, and a person born in the slums of India could fight out of their poverty tooth-and-nail, and maybe achieve a success that would appear to be middling, when viewed from middle-class America... yet in so doing be more virtuous and heroic than an apparently successful socialite who was born to wealth. (If you're familiar with The Fountainhead, Roark in the quarry was far more admirable than Keating at the top of his profession.)
As for evaluating one's self, there are questions that only you can answer: are you doing the most with what you have available to you? Do you strive? Work hard? Are you as honest with yourself as you can be?
Being an Objectivist is not about being a Titan of Industry, or bringing the most value to society, and being wealthy or apparently successful is no sure sign of morality. It's about being the best you can be (in your individual context/situation) for the purpose of leading the best life you possibly can.