Editors of TV current affairs programmes believe, probably correctly, that guests who can be relied upon to say outrageous things are a sure-fire recipe for making their shows more entertaining - and may even help to increase their ratings.
In reporting on this week's riots, BBC's Newsnight brought quite a few such 'experts' to our screens, including former Sun editor Kelvin MacKenzie and historian David Starkey.
Last night's performance by Dr Starkey (which you can watch in full HERE) set Twitter alight for a quite a while, with accusations ranging from claims that "he's a 'racist" to "he's bonkers".
I tweeted that I don't think he's 'bonkers' because it's difficult not to be impressed by the way he's managed to carve out a niche for himself as an all-purpose rentamouth. Like Mr MacKenzie, he can always be relied on to say controversial things that will shock, irritate or amuse a substantial proportion of any audience that happens to be watching.
For Starkey himself, an important spin-off of his 'celebrity' status is that he presumably sells far more history books than most professional historians.
As for how good a historian he is, I have no idea, as I've never read any of his books.
I also have no idea where he gets the idea that the English "don't make a great fuss about Shakespeare" (Question Time, 23 April, 2009), unless he went to a very different school than the ones most of us attended.
Nor am I convinced by his glib dismissal of Robert Burns as a "deeply boring provincial poet".
Whether or not his performance on last night's Newsnight exposed him as racist about black members of our community, the above clip shows that he not only thinks that it's jolly funny to make racist-sounding noises about the Scots, Welsh and Irish among us, but that he also revels in the boos and laughter his calculated insults attract.
Most depressingly of all, such 'entertaining' episodes inspire the editors of Question Time, Newsnight and other current affairs programmes to inflict him on us again and again and again...