A large number of economy passengers don't fly regularly, but when do they wish to escape the 'mayhem' of the airport for a quiet lounge with snacks and drinks (at least according to threads on other forums, trip advice websites etc).
Of course, they are also unlikely to chose an airline / airport based on lounge availability - I'd assume they'd chose whomever was closest or cheapest.
T5 really isn't a bad airport to relax in before a flight.