I lived in Miami for many years and I don't think a MLS franchise can be successful in that town even with Beckham's backing.
In the past MLS franchise teams have developed out of smaller teams from USL-1 the 2nd tier of US soccer. Following the NBA expansion model, the team/organization had to demonstrate that it had enough local interest to support a MLS franchise. It's my understanding the Miami/Beckham plan is to create a team out of thin air.
Perhaps the MLS think that bending the rules for brand Beckham (again) will be successful but there is scant evidence to suggest that Miami has fans willing to regularly show up for soccer games.
It is well known that South Florida in general has 'fair weather' fans - see the Dolphins, Marlins and the Miami Heat. It's difficult to predict exactly what the largely transient population of Miami will and won't show up for.