Please can you tell me if it is okay to leave winter tyres on all year round? I have read that they out perform 'normal tyres' at below 7 degrees, but do they wear particularly quickly at higher temperature?

I realise they are expensive, and it may be a waste, but I drive such a low mileage(about 5000 miles a year) that for safety sake, it may be better not to store an old set of tyres and re-use them.