Well, in reality any idea that is not supported by evidence is merely that: An idea or hypothesis. A theory however dissociates itself from a hypothesis in that there is evidence available to support its premise and that the evidence is correct or has been tested to be correct.
So, this being said, the need to support a theory with evidence is imperative so that it can eventually be accepted as a law. If you think about Troy, for example, it was long thought of to be a mythical city until evidence led to its discovery. In that case, evidence was imperative. In cases such as scientific hypotheses, it would be hard not to have evidence to sustain a claim or to guide an investigation. It is the foundation that would establish the investigation.
Evidence is information on ascertainable facts which, in combination with logical deduction, prove or disprove the validity of a statement or assertion.
The amount and quality of evidence available can be described in terms of the extent to which the proof provided by it is disputable. It will be ideal if all the scientific theories and historical knowledge was supported by evidence that was absolute, ruling out any alternate views. But the reality of science is not that simple. The progress of science has been a series of steps that have replaced one existing theories by newer and better ones.
In practical terms, we can say that the evidence should be enough to establish the truth of statement based on it. But we need to realize that the task before us is not of finding enough evidence to support our existing beliefs and theories, but of continuously finding additional evidence to refine and improve our understanding. No amount of evidence is enough, beyond which no evidence is required for further development of scientific knowledge.