Generally, if an idea is common knowledge, there's no need to support it with evidence, because everyone already accepts it as truth. But if an idea is not universally accepted (or even at all), it needs evidence or credible sources to give it substance, or your audience has no reason to believe you over the other guy.
What is really hampering new ideas is the tendency of people to accept ideas that lack evidence or credible support, preventing new or different ideas from gaining ground.
Unlike what Schicksal said, common knowledge without evidence is not enough. For thousands of years, it was common knowledge around Europe that the earth was flat and the sun revolved around it. This "common knowledge" kept the truth from coming out to the masses. Even when evidence was found, to teach the theory was considered blasphemy. At the time, blasphemy was against the law so if convicted of it you could face punishments including death.
As far as needing proof, no, the people that develop science almost never have proof till they follow where the evidence leads.
Yes, using sources is helpful in debate judging here. But not all debaters use sources, and even fewer always, and you don't have to use sources if your opponent isn't. For example, I myself don't use sources by default, but I'm usually more than happy to provide some if my opponent offers any. But OP, what do you mean by "entirely new idea"?
I am me, and i must cite the source in debates or else some undiscerning voters will penalise me for not giving my source. I refute some very simple points using common logic and i must give my source. I propose an entirely new idea and i must also provide evidence. Very Rational.......