First, Messrs. Smith and Wolf, let's address personal safety. No question whatever, the existing MIA/HIA facility is located "uh, not in the best" part of town and anyone going off its secured by fencing premises on foot - especially laden with baggage, is taking a personal safety risk. While I have in the past walked from there to the "within sight" Metrorail station without incident, that was in broad daylight and without baggage, I wouldn't be about to nowadays considering the "not so people friendly" times of day (well OK, #92 Star) the trains depart and arrive.
So, in short, if passenger convenience were first and foremost, the move should be made.
But where does passenger convenience stand in a market that Amtrak shows little interest in attempting to promote when set against operational efficiency? The LD's largely belong to the die hards (not necessarily railfans; OK, I did meet a nice young girl at DFB a year ago who goes to Univ Central Florida near ORL and she said this train is convenient), and the only way a move could be justified would be is if additional revenue arises from having the trains serve MIC over MIA/HIA in excess of the additional costs (facility rental, the highway infrastructure changes for which Amtrak reportedly will be on tap, access to the FEC's Iris X-ing, security, et magna alia) that will be incurred.