Depends on the source and the kind of research being done. Sometimes, a sex joke scratched into a wall by a Roman legionary is more useful than any text written by a biased Patrician. The same way, future historians might be equally interested in PerezHilton.com and the New York Times website (which has a historic archive stretching back to the 19th century is already often used as a primary source). It depends on what you're researching. Further, not all primary sources are magically good. In my own area of interest, there is an eleventh-century bishop of Cremona that gives an extensive contemporary account of the Papacy. However, historically Cremona always was an Imperial-Leaning city, so any use of said source must be taken with a grain of salt.