I recently discovered that my mother has been doing exactly the same!

She never used to spend her Sainsbury's vouchers. I think there's a difference in mentality between requesting vouchers at the checkout (as with Sainsbury's) and automatically receiving vouchers in the post (as with Tesco). The Tesco vouchers come in the same mailing as a load of discount vouchers, which gives the recipient the impression that they should be spent rather than saved.