Standing up, worth practising for a casual cyclist? I have no interest in racing, for me a bike is a mode of transport or a pleasant way to enjoy the countryside.

Currently, the only time I ever stand is to accelerate from stationary or for a very short, steep hill... I can't do it for more than about 30s and it ruins my legs!

Friends of mine say they never stand, us it something that's useful to practice or really only for racing? Does it perhaps use different muscles so can be used to alternate with sitting on a hilly route or something, or is it fundamentally a hard core skill?
As you have found, pedalling out of the saddle can generate more power than remaining in the saddle, but it is also a more difficult position to maintain. The gains in power is why you see the technique frequently used in racing, but that doesn't mean you can't use it for transportation or casual cycling. In fact it can be beneficial as the human body does not respond well to being restrained in one position for long periods of time.  Interspersing periods where you pedal out of the saddle can be a good way to allow your body to change positions, relaxing some of your postural muscles and allows different leg muscles to be used.

You do not need to ride long periods out of the saddle to gain some of these benefits. 30 seconds out of the saddle is not a trivial amount of time.  I would suggest keeping the period of time shorter if you find your self tired after 30 seconds, but try increasing the frequency of times you ride out of the saddle.  This is will help build endurance, without having to turn your legs into noodles.


  Does it perhaps use different muscles so can be used to alternate with sitting on a hilly route or something, or is it fundamentally a hard core skill?


I wouldn't call it a hardcore skill per se, but hard core riders will use the technique more frequently.  Even if you are not "hardcore" (whatever that is), it can still be useful to more casual riding.  Perhaps you have go for a longer ride than usual, and you are finding you are having difficulty on a hill, standing can help you get over the hill. Or perhaps you are carrying more weight on the bike than you usually ride with (e.g, a big grocery run), pedalling out of the saddle can help you to get moving and up hills you may encounter.