Al doesn't need to win any primary. The Dems apportion their delegates by percentage of votes received. The winner needs to get 2170 delegates to win the nomination. With so many running it's going to be difficult for any candidate to get there.
The Beauty for Sharpton is he needs to only win 15% of the Dem vote in any state and stay in the race until the convention. With the south's black population anywhere from 20-30%, Sharpton has got a real shot at getting 15%. Remember 20-30% black population equals a potential 40-60% dem voters. Add some Republican voters who want to play in the primary, too. Cross party primaries are not just for Dems anymore.
If the Dem race stays tight, Al's delegates will allow him to be a player and extort favors and prime time coverage from whoever he gives his delegates to at the convention.
Remember, Al just needs at least 15% and to not drop out to have convention delegates. I think he will get some. D.C. wants to have an early primary. S.C., MS, MO, FL, LA all have fairly early primary dates. They could give Sharpton the momentum in other states such as NY, NJ, MI etc.