The arrest allowed Crowley to make a full report and document everything that happened. I think Crowley may have a tape recording of everything that occurred. He seems like he's the kind of cop that would want everything recorded, just in case he ran into a race baiting @$$#0!E like Gates.
That's something that occurred to me, too. I'm sure Gates would have screamed racism no matter what, and the fact that the officer arrested him kept everything out in the open.
It is standard procedure to bring somebody out of their house (to the porch, in this case) in certain situations (out of control domestic call, for example), both for their protection and that of the officer. In this case, the fact that Gates was publicly visible prevents him from claiming he had been roughed up or abused in some way, and arresting him certainly makes everything very public and above board.
Otherwise, I'm sure Gates would now be claiming police brutality.
That tape is on its way to Hollywood for editing/photoshopping
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Crowley would have filed an incident report with or without an arrest.