Everyone has a personal agenda.
My belief is that there are three key questions to ask yourself about a source. 1) It is important to determine where sources are coming from, 2) if they have a bias toward the topic your story is about, and 3) what their intentions are in answering questions. Of course there are people out there who use media outlets for personal use (getting a specific message out, lashing out against others, or just trying to get their name know) and it's pertinent that as journalists we know who we're talking to.
I've dealt with sources before who have tried to make me incorporate one thing into my story, when in the first place the questions I was asking had nothing to do with their own agenda for the story. As a journalist I feel it is my duty to give information to the public that is informative and newsworthy. Some stories stem from topics that lead to sources who obviously do not have the credibility worthy writing about. Therefore, we need to seek out credible, trusted sources who have the same values to give informative insight to the public.
With the popularity of online sources now it's also necessary to go back to an online's original source and make sure it's credible. With blogs and such this can be hard, which is why I think it's just as easy to seek out a living person, or a representative of a source to use as your quote.