Anvils aren't exactly the biggest use of iron. Think rails, pistons, etc. EDIT: Also, from a bit of experience, the blockdata glitches aren't reliable, likely to get patched, and don't work as well on EMC as they should. Also, it's a exploit, soo...
also i hardly think this would break the iron economy. most big shops offer free anvil use anyways and theres tons lying around emc