I'd have figured anyone writing a piece on this would spend more time demonstrating how Clinton's likely easy primary win can help in a General Election. There's a case to be made for that, but the piece ignores it.
I think it's more addressed to the ridiculous media concern trolling that continually oscillates between "hilery not inevitable!!!1!1!!" and "hilery is inevitable but it sux 4 her". They could at least make up their mind.
Quite frankly, I find the whole concept of "needing" a primary to be patently ridiculous. And the only time people try to apply it is to Hillary Clinton. Why wasn't the media talking about how Obama should get a Democrat to run against him in 2012 to "make him a better candidate"? What about a Republican against Bush in 2004? Primaries against incumbent Senators, governors, or Representatives are universally treated as a bad thing for the candidates in question. And for good reason. Not only will the person have to use resources fending off the primary challenge, they could also, you know, lose. Which wouldn't be a possibility against no opposition or token opposition.
You can make the argument that it's a different situation for a non incumbent, but that still doesn't work. For one thing, there's the fact that she's essentially the de facto incumbent anyway. She's certainly polling like one. Secondly, people see clearing the field as a positive even for nonincumbent candidates. If you need examples, just look at 2014. You didn't see the media concern trolling about how Tom Cotton and Cory Gardner's uncontested nominations left them "unprepared" for the general, instead you heard about how it was their strength as candidates that allowed them to clear the field. A challenge to Gardner from Tancredo or Buck would've been universally seen as a bad thing. Well guess what? The same thing applies to Hillary.
It is a bit different.
The expectation is that an incumbent will not be challenged in a primary, so a serious challenge would suggest that the incumbent is weak somehow (even though it's not clear the extent to which the primary makes the incumbent weaker, or weakness makes primary challengers likelier.) Incumbents have a high profile due to their positions, which they're able to utilize for free media.
Candidates in active primaries will get a lot of media attention, thanks to debates, campaign events, interviews, etc. If Hillary's keeping a lower profile, there won't be as many challenges to the Republican arguments, and there won't be as much coverage of the Democratic arguments.
The concern is that progressives might get complacent if she's coronated before the primaries even begin. 2014 taught us how important turnout is, and we can't just assume turnout will naturally be higher in 2016. Clinton needs to start campaigning long before the GE, and give progressives a reason to turn out and vote. Point to her favorability rating all you like, that doesn't mean people will actually show up to the polls.
It's honestly not clear. She currently leads in polls, partly a result of her less partisan image. There will likely be a decline during the General Election, so there's an argument for keeping the election period as short as possible. But maybe she'll be better able to weather the decline better if she's in the race longer.
In the event that a progressive candidate lost with a signifcant chunk of the vote (say 30 percent), she'll also have to deal with some potentially pissed off base voters.
It could also be easier for her to stick with arguments against Republicans for as much of the campaign as possible, whereas Republicans will have to switch from appealing to primary voters to the General Election campaign.
I think there are pros and cons to a competitive primary, and I honestly don't know which would be better. There are all sorts of odd factors. For example, there's the impact of states where independents can vote in primaries is unknown. Maybe centrists who don't have the option of making a difference in a Democratic primary will pick more exciting/ moderate Republicans.
