Theodore Roosevelt, Calvin Coolidge, Harry Truman, Lyndon Johnson, Richard Nixon, Gerald Ford, George Bush (Sr) are all twentieth century Vice-Presidents who made it to the top. So it's actually quite an important role in US politics, frustrating as it is for the post-holder. There are some huge names in that list- Teddy Roosevelt, Lyndon Johnson, and Richard Nixon obviously stand out (not all for the right reasons.) You get there by your predecessor finding a stray bullet or meeting his maker. So you can understand why Lyndon Johnson felt like a raven on JFK's shoulder.
John McCain must choose wisely. His running mate will be a huge issue. It will be less of an issue for his opponent.
Just a salutary thought but imagine if George W. Bush hadn't made it to the end of his Presidency. He has been an abysmal President but can you imagine Dick Cheney?