Soft evidence can refer to anything from simple word of mouth or argumentation, to authoritative opinion on a given subject. The highest form of soft evidence tends to be supporting opinions from an authority with certifiable credentials. While random guessing also qualifies as soft evidence, it is generally not worth pursuing given that even the highest form of soft evidence are still just that: soft.
Soft evidence does still have value, but only in the absence of hard evidence. If there is no supporting hard evidence, then the best available forms of soft evidence should be used in the meantime. A further problem with soft evidence is the appeal to authority.
While most of the time an authoritative opinion will be valid, it is not always the case. There is a danger of placing too much faith in an authoritative source, as it is important to remember that even though it is the highest form of soft evidence, it still does not outweigh hard evidence.
Hard evidence is the product of producing testable predictions, performing controlled experiments, relying on quantifiable data and mathematical models, a high degree of accuracy and objectivity, and generally applying a purer form of the scientific method
Gary: I assert that the supernatural claims of Christianity are based entirely on soft evidence.