giffgaff are a buisness and have to show a profit. Your idea has Ben suggested many times, and as mentioned in previous posts Idea's have be submitted. I cannot se giffgaff adopting it as it would lose them money.
Its also the same with the £5 hokey plan. I only use about an average of just over 100 texts per month, and always have minutes left over at the end of the goody bag expiration date. I cant see giffgaff refunding me credit to use on my next goody bag.