5 minutes' reserve, as a tactic to get air traffic controllers to put some other airline's arrival into a holding pattern, is all very well, until the other airlines begin to follow suit...
"On July 26 last year thunderstorms over Madrid led to the serial diversion of flights to Valencia and three Ryanair aircraft were among four to request emergency landings because they had reached the limit of required fuel."
Full Article:
http://www.travelweekly.co.uk/articles/2013/07/22/44737/ryanair-probe-prompts-call-for-aircraft-fuel-levels.html
In the BBC program (a month or so ago) it was pointed out that they like to fly to airports 40+ miles from where people are trying to get to, so as to save on landing fees and not have issues of landing delays. So I see it as a case of 'horses for courses'; low fuel works well for quiet destinations but they need a policy of proper reserves for flying into major cities.