It used to be that the conventions actually chose the candidate. The last time the nominee was in doubt at the beginning of a convention was 1972 when McGovern and Humphrey argued about California's winner-take-all rule. If the convention voted to change the rule then Humphrey would be the nominee. McGovern's side won.
For a while the convention continued to have a little suspense. The candidate did not announce his running-mate until after he was nominated. In 1980, Reagan was still negotiating with Ford to be his running mate. That collapsed during the convention and he chose Bush instead.
These days the convention mainly serves as an infomercial, allowing the party to introduce the candidates. There is still a little drama. In 2008 this was the first real look that most people had of Sarah Palin. For Barack Obama, the sense of history was so great that he rented a stadium for his acceptance speech. The convention wasn't big enough for the moment.
This year will be less exciting but it will still be the first time that most people get to hear more than a soundbite from Romney or Ryan.
Romney will certainly get a bounce in the polls from the convention. Obama is a well-known commodity so he will get much less of a bounce.
The big question is how big and long-lasting each candidate's bounce is. There are three possible outcomes:
1) Romney takes the lead and holds it until the election. Neither candidate has broken 50% yet. The undecideds know Obama but have not made up their minds about Romney. If he can connect with a substantial number of them then he will win the election.
2) Obama takes the leas and holds it until the election. Obama will need to remind people of the inspiring candidate that he was instead of the political hack that he became. If he can reconnect with the independent voters again then he will win.
3) Neither candidate's bounce lasts. With the candidates polling within the margin of error, the election will hinge on who makes a major slip. This could happen in the debates or a major foreign crisis could come up (Israel attacking Iran is the most likely). Romney has an edge in the debates since he won several of them during the primaries. Obama has not debated since 2008 and may be rusty. If a foreign crisis erupts then it is out of Romney's hands. Obama will succeed or fail on his own performance.
We will know which of these possibilities will determine the election by mid-September. If a post-convention bounce is going to fade it will do it by then.