I agree with Maskers-87126 that Ben Cooper's Breckinridge character was basically an audition to be a semi-regular character. I think he was contemplated as a replacement for Quint, whose last episode was shown the next week but which was filmed before 'Breckenridge' and "Two Tall men', the two episodes where Breck appears. I don't think that Carl Reindel's 'Cale' character from three seasons before was intended to be a semi-regular. I think that that character, a wanderer, might have been under consideration for a series of his own. (There's a couple other episodes from that era that looked like pilots).
The problem with Breck is that there wasn't much he could do with the character. To have an attorney as part of the series turns it into "Arrest and Trail" where Matt would arrest somebody and a sympathetic character would then try to prove him innocent. The audience wouldn't know what to think.
Thad would have been a much more interesting character if they'd hired Jon Voight, a much superior actor, to play the role. Roger Ewing was a nice, good looking guy who couldn't act so Thad was a nice, good looking guy who wasn't interesting.