This seems to be the approach that a number of businesses are trying, but the goal behind the IPv6 design was to prevent all but peering-sized companies (e.g., Google) from getting provider-independent blocks to reduce the size of the global routing table.
IPv6 hosts are required to be able to handle multiple addresses per interface, and the intention was for multihoming to work by having the enterprise's egress routers each advertise the block (generally a /48 or /56) available via its uplink, and for the routers inside the enterprise to append the global prefix (generally read via DHCPv6) to a prefix-independent subnet number. Migrating hosts that are getting their information from router advertisements can be done gradually and without admin intervention.
Unfortunately, in actual deployment this model was hampered by the adoption of the
AAAA DNS record (which stores just a literal IP address) over the
A6 record, which allowed specifying address components (e.g., an enterprise-wide 48-bit prefix part and an 80-bit host part) that could be managed and update independently; and by flaky prefix-based address support in early router software versions, and it seems rather unlikely that the multi-address model will gain any traction over the PI+BGP model. Early RFCs recommended against assigning PI blocks to non-transit organizations, but as of at least RFC6177 this recommendation seems to have been withdrawn.