That would return politics to the grass-roots level. Candidates could politick in person at the conventions. It would cost very little. As it is now in most states, candidates have to spend as much on primaries as they do on the general election. Even though candidates are trying to reach only members of their own party, there are no radio, television or newspapers ads that only go to party members.
Candidates have to buy ads that reach the whole population.
If there's a runoff, then candidates are forced to fund three full election cycles. That's why elections cost so much. Primaries in which political parties choose their nominees came out of the Progressive Era. It's one of several reforms from that era that has proven itself to be more of a problem than a solution. The idea was take politics out of the smoke-filled back rooms, to use that old clich , and let the people as a whole choose their nominees.