User blog comment:Donut4/ANOTHER QUESTION!/@comment-306173-20120327174710

Because Arthur would help them and join them, possibly ending up on the run with them afterwards. All three of them, Harry, Ron and Hermione, know Arthur is important to keep the rest of the Weasleys together. Not to mention how it would raise suspicions to see Arthur with Runcorn -- Arthur is supposed to loathe Runcorn for what he does at the Ministry.