I believe that we should use our profile to include the major key words we use to identify and promote our 'key' hubs. By key, I mean those hubs that are obviously in niches that we want to emphasize as our main areas of concentration.
It seems to me that Google searches our Profile to determine where/how to rank us and if we have cohesion with our profile and our hubs we look better to Google.
I am not quite sure what you mean by credentials. My credentials don't help me with what I am writing about now. So I emphasize what I like to write about, including mentioning the names of some of my hubs.
Interesting question. I think it has much more in it than we might see at first glance. In other words, it might make a good forum thread.