I guess my problem is that you're defining journalistic contribution to society as merely 'being trustworthy', nothing else. Nothing about information dissemination or airing of issues. The sole contribution. It'd be like saying the sole contribution of police to society is looking recognisable by wearing a uniform.
The sole contribution of the journalist is the value that the reader extracts from his writing. In the highly specific example of a person writing a product review, his ability to give an honest assessment of the product is obviously the most significant concern.
In a more general sense, honesty is extremely significant in nearly every sort of non-fictional publication. As you said, there are some cases where fact-checking is either unimportant, or outright impossible, I didn't mention it because that's the sort of thing that should be taken for granted.
The sole contribution of the journalist is the value that the reader extracts from his writing.
This is overly idealistic. For example, journalists are quite adept at massaging public opinion for vested interests, often without the reader being aware that this is happening.
