Has this ever been suggested? I think it would be popular choice for a lot of members.
Popularity does not always equate to sound business sense or management.
After you remove vat and banking charges a £5 top up leaves giffgaff with around £4 to profiteer from. The £5 airtime could sit there for years apart from a six monthly text to keep the sim active.
Luckily, giffgaff do allow the £5 purchase of a goodybag that expires after a month.