I think if you have closed Primaries you run the risk of group think and a drift towards the activist wings of your party. Which given Single Member Plurality voting leads to a lower house full of fairly outré characters.

So the counter balance is to open the Primaries so that ordinary people can have a say in selecting the candidate, which leads us to your point.

Someone who is a keen partisan is probably more likely to vote for a nut-job incompetent in their opposition’s Primary than an ordinary voter.

I think in the UK one is more likely to have sufficient ordinary voters turn up to vote than partisans of the opposition and therefore Open Primaries in the UK are I think more likely to produce

Of course the problem would go away if we used preferential voting.