Although there's no reason that the other (non-noisy/faulty) side should be changed it does however make common sense to do it at the time, the same as you would a water pump when changing timing belt for example. However a good enough reason is 'my car will soon be out of warranty and your top strut mountings are sh*t so please change them both considering I spent over £10k on the car and I really can't be bothered quoting the sales of goods act which i'm sure you already know'