actually size was a consideration and a better locking fit. The two sprint teeth on the underside help keep the plug engaged. There were other reasons, but those were the two I remembered when reading about it. It does allow for slimmer devices even if it doesn't seem like it matters to the Evo, but don't forget the HD2 which is slimmer than the Evo. I miss their proprietary modified USB connector that had a pinout for audio.
And don't forget devices like BT headsets and other small devices benefit from microUSB.
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