There have been the rare time where the "Auld Crabbit" has been used in a rather condescending manner. In much the same way a skilled mechanic might speak to a backyard mechanic, saying something like " Well you can do this repair yourself or let a pro do it. But what do I know I'm just a mechanic!?"
Used this way it indicates that the person knows much more than the backyard mechanic and is showing so but giving his advice then offhandedly tossing out his status as a proffesional.
Once and a while during a thread someone has offered lengthy words on how they're oppinion is the right and true oppinion ( or something to that effect ) and then tried to "dumb it down" by saying " but I'm just an Auld Crabbit". People reading the thread know that the author is coming from a level of authority ( warrented or supposed is another matter ) and it's obvious that the advice, critique, or what have you is meant in all seriousness. When the author ends with " But I'm just and Auld Crabbit" They are implying that they are in the know and their words should be taken with utmost seriousness.

Now of course there are times and I think it is most of the time that the title is tossed out there just in fun. I am simply pointing out that every once and a while it is not and maybe that is where the OP is coming from.