No, it because of a variety of reasons:
Brown is still well regarded amongst a section of the Scottish electorate.
Labour voters, especially those who regard Brown well, are those believed to be moving from 'No' to "Yes'.
English politicians are loathed by that section of the electorate.
Therefore, get Brown to lead the charge, providing more comfort to the people they want to win over.
Maybe, perhaps maybe, you aren't the intended recipient of the message?