Since when? You have weighed up the cost of running the heating for the radiators to be on all the time? What about in March/April even summer when it's rainy and you can't use the washing line and it's too hot to have the heating on. Tumble dryers are convenient.
Yeah just been looking myself, seems they arn't bad to run these days,
http://www.sust-it.net/energy_saving.php?id=41
So saving approx £20 a month, £5 a week or approx 71p a day to have cloths drying off in the air, making the area damp, take an age to dry, bedding being a nightmare to hang around the house....
Is it me or does it not seem worth the saving?