I have one (but it was much more active when I was younger) and I found it very useful to go through reasonings. However mine is not critical of me, it stops at pointing logical flaws in my reasonning (as others said, it might be a matter of self esteem).

Speaking with people who write, I found that roughly half of them can have a litteral chat with their caracters in their head, me included, I would not be surprised if it were correlated (people who cannot where chocked that it could be possible).