Despite supposedly inventing the concept of a credit card (or charge card, to be more accurate), holding a Diners Club card is fairly pointless.
However, one of the perhaps lesser-known perks of the card is that it entitles you to complimentary access to airport lounges. We’re not talking about First Class lounges for flagship airlines, but rather generic independently-branded lounges. You’ll typically find that you’ll get a more comfy area to rest, free snacks, drinks (including alcoholic), wi-fi, and sometimes showers and other amenities.
With lounges located in all major airports, obtaining unlimited free-access for £50/year (the annual cost for a Diners Club card in the UK) is a no-brainer for a frequent flyer. If you conservatively value all of the above for £5, you’d need only fly through an airline 10-times during the year for it to make sense. AMEX offers a similar perk on with a Platinum card costing £450/year, whilst the similar access-providing PriorityPass card costs £259/year.
Update: Diners Club has effectively closed this little perk – they now charge £15 per lounge visit, which makes it fairly worthless unless you have a lot of hours to kill at the airport.