Its possibly a reflection of th e fact that banks see themselves...

Its possibly a reflection of th e fact that banks see themselves and their possible target acquisitions as so grossly over-valued that to pay market value for a competitor would be far harder to justify than simply buying their own shares.

At the end of the day though it is shareholders own funds being used to buy their own shares rather than grow the business and artificially provide buying support.

It must act to magnify potential upside movement if things go well as fergle points out but has the same effect for downside too.