Let's be clear from the outset. If we want to find out which, if any, of the three candidates with a realistic chance of becoming leader has the substance required to win over the electorate in 2020 then there needs to be a debate.
Three people standing there saying the same thing in a slightly different way is not a debate. It's a beauty pageant. The 2020 election won't be.
Like it or not there are a substantial number of people within the Labour party positioned on the left. If the party tries to sweep them under the rug and pretend they don't exist then the public will see through that. It will look like the party is trying to hide its true face and will give the Tories further ammunition come the next election.
If you want to win Tory voters over then you have to prove to them that Labour has changed. The way to do that is through a public debate where those in the centre defeat and are seen to defeat those on the left. If any of the three main candidates really wants to prove their credibility to the nation then they now have an opportunity to do so.
In fact I'd be willing to bet that whoever does best against Jeremy Corbyn in the hustings / debates will be the eventual winner. At least I hope that's what happens, because if the next leader can't win a debate with those on the left what chance will they have against the Tories in 2020.