You lose the economic benefit of the plutonium, the spent fuel remains radioactive longer and has to be better guarded, because it contains plutonium. If the reactor design is much more economical of neutrons, enough U-238 can be converted to plutonium so that after a fuel cycle there is more fissionable material than there was in the original fuel rods in the reactor. Such a design is called a breeder reactor. The billion year estimates for fuel resources depend on breeder reactors. The French built two of them, the U.S. has a small one, the British built one, the Russians built one and the Japanese are building one.
Breeder reactors seem to be a resource rather than a reserve.
They are more expensive than present reactors and will probably not be deployed on a large scale until uranium gets more expensive. This is unlikely to be soon, because large uranium reserves have been discovered in recent years. This was a breeder reactor with reprocessing on site, so no plutonium ever became externally available. It was hoped that it would address the proliferation concerns of the anti-nukes, i.e. it was hoped that they would be appeased. However, as soon as the Clinton Administration came to power, its anti-nukes got the IFR cancelled.