Hi Slash. I believe in momentum as far as it is something that exists. I don't believe in it in terms of having any predictive value.
For example, if a team has won 7 games in a row to end the season, that team has momentum. If, however, one said that this team has a better chance of winning their first postseason series because they have all the momentum, that would be a false statement.
The team with momentum is not more likely to be successful in the future, whether you're talking about one play, one inning, one game, or several series.
In other words, momentum is one of those descriptive words of what has already happened, not a predictive word of what is likely to happen.