That question was asked of the Marine Corps after the two World Wars, Korea, and Vietnam. After each conflict there were many who discounted the utility of the Marines, citing cost-effectiveness, duplication, and myriad other reasons as justification for the elimination or absorption of this singular and peculiar organization. But the Corps survived and justified its existence through its performance in and out of battle. Nonetheless, it will face renewed scrutiny after Iraq and Afghanistan and the result will be the same—but only if the Corps remains useful and does what it says it can do.
In the public's eye, Marines have been almost indistinguishable from the Army for the past five years of the Iraq War. That was also the case in the wars cited above. But the Corps was born to serve on the Seven Seas and that's where its future will again reside.