While I’m more or less resigned to accepting the 2016 Democratic coronation, I do think some kind of competitive primary for the nomination would be valuable, even if the outcome didn’t change. If for no other reason than that it preps the nominee much better and keeps them from getting rusty while waiting for the other party to get it together.
It seemed “bitter” at the time, but the 2008 primary on the Dem side was one of the best things that could have happened to the party as a whole or either candidate, regardless of who had ended up winning. We never could have won Indiana and North Carolina that year in the general election if the Dems hadn’t been registering people through May and June there during the primary battle.
And on the flip side, we can look at the 2000 Democratic presidential primary. Vice President Gore was guided strongly into the “inevitable” position by President Clinton (whom he then tried to run from, which was weird, given Clinton’s continuing popularity at the time). Gore’s only challenge was a very weak run by Sen. Bill Bradley (NJ), who is a good guy but had no real chance of prevailing.
This meant Gore — who hadn’t run for office in his own right (i.e. not in the running mate slot) since the 1988 presidential primary — essentially didn’t campaign seriously in 2000 until about October. Then he suddenly woke up to the fact he was about to lose and then he campaigned like crazy. He was actually pretty good at it, and appealing, in the final weeks, by most of the accounts and polling I’ve read (since I was a bit too young to notice most of it at the time). But it was too late in the Electoral College, popular vote victory and Florida shenanigans aside. That he even came close enough for it to be stolen from him (if indeed it was) is a miracle given his lack of campaigning until right near the end.
(Exception to the above: Incumbent presidents tend to be hurt by primary challenges, though it’s unclear if that’s because they’re only challenged when already very weak, but they are already in full-time campaign mode anyway and thus don’t need the practice a non-incumbent requires.)