The runway behind you, the space above you - and fuel left in the bowser are all as equally as useful!
My previous employer had a policy of taking flight plan fuel wherever possible. A Captain could use his discretion to carry more but had to justify it in writing on the flight plan. I was amazed at the number of captains who tried to graticiate themselves to management by carrying flight plan fuel even though the forecast was saying otherwise, eg thunderstorms in the New York area. I averaged a fuel diversion once every 9 months in that company.
My present company would like us to minimise fuel carried, but doesn't take anyone to task over it. Despite flying in the tropics during the monsoon, I have not had one fuel diversion in three years. I can't be bothered to work out which policy is cheaper -but I know which is safer.
I think the advice to carry 20 minutes is sound and should be heeded. In fact, I'd carry 40!