Labour will continue to benefit from their open leadership contest. It gives the media an excuse to pay otherwise unwarranted attention to their favoured party. That attention positions Labour as a material influence in politics when they should objectively be ignored for the next year or so until they have worked out how to stand for something more than reactionary envy and 1970s policy.

But I doubt that the membership primary will add much internal value.  Whether Labour emerges with a leader who can stimulate, tolerate and then carry the burden of a major ideological shift will be much more the result of coincidences of personal loyalties and ambitions than of any educative effect of internal debate and exposure to set piece oratory competitions before the members.

Internal candidate debating competitions are deeply flawed exercises in democracy. They cannot perform the main function of an election campaign. They cannot explore and expose the critical weaknesses of the candidates. If they did tease them out they would wound the party privately and publicly. Little damage is more long-lasting than the damage from publicised frank assessments of weakness by close colleagues.  So intraparty primaries become competitions in public self praise, with little risk of contradiction. Sure there may be cunning allusion, comparison by emphasis. But formally the candidates can only highlight their own attributes, and stay away from the dangerous territory of exploring their competitors' weaknesses.

The contest before members (and the media)  may allow members to compare oratical skills and to assess charisma. But oratory and charisma are desirable, not essential. A leader's greatest influence over the longer term will be on the party's internal culture. Does it value wisdom, courage, loyalty, honesty? Or does it  reward more slyness, ruthlessness and skill at playing the game.

If the candidates have any suitability for leadership at all they know that the party can only address those questions privately despite them being the most important. So intra-party primaries are  boxing with one hand tied behind, decision making with the most important information hidden.

Because Labour has not made the mistake of leading party members to think they will make the decision, the caucus voters can still test and draw out of the woodwork the evidence of worms and knotholes and character fungi.  They will not be forbidden territory in private.

But that means the eventual decision could run counter to the way members would vote after observing their oratory competition. The consequence can be unjustified rank and file suspicion of their representatives. 

The bigger risk for a party in running a primary is also unlikely to emerge in the current Labour exercise, because the decision has not been foolishly given to party members who go to the competitions. The bigger risk is that one or more of the candidates will decide that members must know of their competitor's defects. It may be just the short term interest in winning. It may be supplemented by a sense of duty to ensure that the voter decision is properly informed. Either way means the airing of dirty linen. All people and all organisations have some linen that is at least smudged.

A ruthless candidate may covertly promote third party questioning of competitors to elicit public exposure of vital negative information. It is a high risk strategy. It can rebound directly  in condemnation for all associated. Many contenders will have been  complicit for years in covering up the worst flaws of their colleagues. Simple loyalty and simple commonsense tell you that your party only thrives when you are all helping to compensate for and to cover for your colleagues' weak spots. So those who put the collective interest first will be hamstrung in responding to a whispering campaign, or even to media questions. The party rule will be no comment, to avoid adding oxygen to the illicit campaign.  One nil to the treacherous contender, at least for the short term.

More importantly the process of "briefing" against a colleague is enormously corrosive. Even if it is done through a third party to a grateful journalist  that journalist nevertheless know that you or your team are capable of treachery. Sooner or later that knowledge spreads. Suspicion that it could be true may infuse your party and warp its culture away from the spirit of altruism needed to make it hum.

To me the US primary process is at least partially responsible for producing parties full of representatives who spend more time highlighting how they differ from each other, and how little the parties have in common, than they do working after elections are won, on what they can build for their country in common. I recognise the benefits of forcing candidates to woo party members. But it should always be clear, as it is for Labour, that the final decisions will be made by much better informed subordinate leaders. Otherwise the costs of primary processes may exceed the benefits. 

I know about some of this from experience.