I hope I'm not doing you a disservice here but it looks very much as though you have a dislike for Kerins and Delaney which is clouding your judgement on this issue.There was no legal obligation on him to answer any questions at any point. But, you question what is the point in showing up and wasting everyone's time if you are just going to stonewall the way he did. You might as well just refuse to attend. So, the AK judgement which he cited, gave him an amount of cover, together with his "legal advice" such that he could say he would just love to co-operate, but he simply can't. It's all about having some kind of story, and selling it to the public.
Suppose for a moment the gardaí arrived at your home and said they were going to search it, would you demand to see a warrant? If they arrested you and charged you with an offence, would you insist on due process and legal representation in court?
Of course you would because you're not a moron. Why then make disparaging reference to Delaney's legal advice and why use the pejorative 'cover' for his assertion of his legal rights?
To address your question about why he attended; I would have done exactly the same. Had he not attended he would have exposed himself to criticism that he was somehow snubbing the state. By attending and asserting his legal rights he was facilitating the committee in its legitimate business but refusing to be bullied when it strayed beyond its lawful limits.
This is why we rejected Howlin's attempt to set politicians up as judge, jury and executioner over us.