Uh-oh.
Well… I can tell you for sure that there are about a million homes in Southern California with 10d hanger nails where there should be 16d sinkers. At one point I did nothing but roll joists for two years in Ventura and LA counties. And I did it the same way everyone else had been doing it for years.
Ooops.
When I was researching the same question Kenton, the answer I found that made the most sense was that the rim joist was not designed to handle the beam load supporting the deck, adding the ledger makes a stronger beam.
Maybe a pic is worth a 1000, find the defects 
(More for your project coming by email.)
This article should help clear up some confusion about hanger nailing. Its always been a peeve of mine that 60% of the hangers I see are installed with the wrong nails, after reading this article I see its more like 90% of the hangers are nailed wrong.
Why a pet peeve.I never see issues of hanger detachment ?
Most major issues are always with support posts and ledgers which is the main cause of failure .
