The fifth star hasn't always had a stellar pedigree. William Manchester's American Caesar doesn't exactly paint a flowery portrait of MacArthur. Fleet Admiral's Leahy and King were political creatures whose fifth star was largely a reward for proximity to power. Halsey made some fairly large mistakes, the largest of which was sailing his fleet into a typhoon and killing a lot of his men. I'm hard pressed to find an exemplary reason for Hap Arnold's fifth star.
A decent argument could be made that a fifth star is more political reward than merit based. As Zen points out, the only functional way Petraeus could wear a fifth star would be as CJCS, the most political position in the military. While I can respect the purist's views that these wars have not been wars of national survival, they've been wars nonetheless.
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