Beale's winning percentage is higher than Cooper's.
Of course it would be great if Beale's defence was better. It would be great if all our players were better at the things they're not very good at.
Based on the statistical improvement in the Wallabies results when he is in the team (which in a 71 test career is not an insignificant amount of data) it can be surmised that the benefits he provides to the team exceed any shortfalls in his defence (and other weaknesses in his game).
I'm not really sure how you can dismiss statistics showing that the Wallabies are more likely to be successful when Beale is in the team, particularly against the All Blacks because you think his defence is inadequate.
It's a bit perplexing.