It is exactly as Elinor says, the only thing Willoughby really achieves in his censure of Colonel Brandon, is by pointing out more of his good qualities as you pointed out. And if the Colonel's 'threat' turned out true, then one can add that he is a good meteorologist too?

And somehow I bet, Willoughby's brown mare is a temperamental animal or somehow unfit for use and that is the true reason why Willoughby wants to get rid of it.Colonel Brandon has too good an eye for a good horse to have fallen for it. I have no evidence to support this statement - I just feel it in my bones (and Elinor may reprimand me for jumping to such conclusions :) )