A weasel word preface used when a person wishes to make an argument or state an opinion while making it look like they are stating fact. If asked specifically about the "some" they've mentioned, the person will usually not be able to list a source of note. (You, by yourself, are NOT a source of note for the purposes of "some argue.")
Most commonly seen on open-edit sites like Wikipedia.
Wikipedia entry: "Some argue that President Barack Obama was involved in the mass murder of over 12,000 kittens in February of 2004."
Sensible person: "Care to list a source for me, buddy? Other than yourself, I mean."