My cynical take is that MLS is where old stars from other countries come to semi-retire. They become big name attractants for ticket sales for MLS teams, but they end up only playing sub minutes later in the games, if at all. If that's what works for them, great. It can be a way to stay in the game and make money (as if they need any more) without the pressure of playing for club and country.
So if Messi wants to come, he can follow in Beckham's footsteps into semi-retirement and eventual club ownership. And Miami, who doesn't seem to have much going for themselves at the moment, gets a star to boost ticket sales and interest.