Conventions are still important for several reasons. First, they help to frame the debate and get people energized. Second, the behind-the-scenes part of a convention is critical because campaign workers come together to cooperate, share strategy, and work out a true solution for the party and not just the candidate. It is also possible that, at some future point during a close primary season, we could have an actual convention floor battle. It is not unprecedented.
I think that it has really come down to a bunch of old guys trying to still prove they are the alpha male. A bunch of guys or women, go into a convention center, tell a crowd of people what they want to hear, in order to influence the vote. But ultimately, once they get to Washington, what they originally promised is not even a thought to anyone that could change anything.
Look no further than Chris Christie's speech at the Republican convention, complete with a three minute rant in the middle of "WE BELIEVE ___" and "THEY BELIEVE ___" as he went down a list of topics. Nobody is swayed by these things, all they are is a get together where everybody talks about how much they hate the other party.
Conventions are really just to get pre-existing party members excited. They are not for attracting new party members. A bunch of politicians speak, and the people of the party get hyped up. People of the opposing party do not care, and undecided voters are more likely to look at the candidate and the issues rather than party hype.