Just as any journalist might spend hours hanging out with a source, developing sympathy and rapport, WL has to use some of the same strategies.
It's a tactic used by detectives, journalists, psychotherapists, etc. It is appropriate in some fields and is part of the professional standard and is not a reflection on credibility.
I do not think we have to look far to assess WL's credibility. When is the last time that significant source material has been provided to some other, competing transparency organization? WL is the market leader and in spite of having been under attack for years has not been left in the dust of some other competitor. In that sense WL has tremendous credibility.
When is the last time someone has leaked a trove of data to the NYTimes after WL sat on it for months waiting for the perfect moment to deploy it? This simply hasn't happened.
All it would take would be one big story where the leakers publicized that Assange said no to the story or that he sat on the information. I think it's quite obvious that Assange actually would publish any significant story at the earliest opportunity.
Snowden chose not to go to WikiLeaks for his own personal reasons and because he wanted to be extra cautious about how aggressively the information was disclosed (to protect national security). Wikileaks is specifically not willing to heavily editorialize the leaked information (by omission or actively) because doing so would create doubt about why he suppressed information, and such doubt would actually harm WL's credibility.
Something like WL can easily be weaponized by state actors, disgruntled employees, estranged partners, etc. Because of that, the decision not to suppress any significant stories is crucial. WL won't publish paraphrased rumors, it publishes the actual source material so that there is no need to make inferences or imagine what was kept back.
It's ironic if not a bit absurd that Assange was asking Don Junior to turn over damaging information about his own father and that his remarks are used as evidence that he wanted Trump to win the election.
On a related but tangential note, it's remarkable to me that nobody has leaked Trump's tax returns yet. I'm also surprised nobody has posted a bounty on Ethereum for the first person to send the tax returns to Wikileaks and publish the decryption key and checksum in a public ledger.
One other thing that I'll mention is that I think that people who like one of the two major parties are much more likely to perceive someone else's behavior as being motivated by partisan loyalty.