Jornalists who hide their sources are usually also manipulated by those sources. How do you make sure as a journalist to get access to high ranking sources within the powers that be? You write the stories they want, or they're not talking to you anymore.
Every journalist will experience politicians and other powerful people wanting to tell them things "off the record". If they enter into those kind of agreements they are also betraying their profession and their audience.
If you are a journalist working with a source who asks to stay anonymous your number one job is to ensure that they aren't lying to you in order to to advance their own agenda.
Obviously they have an agenda, and want to advance it, so you need to figure out what that agenda is.
The next challenge is confirming that what they are telling you is true, to an appropriate level of confidence at least. Your professional ethics and your editor (and your legal team at larger publications) won't let you publish if you can't do that.
There are many ways you can do that - ask them to show you supporting evidence (usually documents) for example - but the most common is to try and find a different source who can confirm what they are telling you is true.
If you can get two sources - anonymous or not - to confirm the same detail and you're reasonably confident that those sources don't know about each other that's often good enough to get to something you can publish.
Unless the source is a whistleblower, their agenda will usually be dirty if they want to be anonymous. And then you're at their whim, because they control the flow of information. If they're showing supporting documents, those documents should be open sourced* to the public or they shouldn't be seen by the journalist.
* As much as needed for the public to be able to verify.
Why are you asking questions whose answers you are not capable of understanding?
An anonymous source has the power to decide what information she lets the journalist have, and thus she controls the exchange. If the journalist does something to displease the source, then the journalist is cut off from the information.
Yes the will always, the difference is that the public will be able to judge for themselves. And people with a different view have somebody to respond to in public discourse.
And it will be a lot different for a named source to defend why she is making public some things and hiding other things.
