I participated in David's bootcamp project (my portfolio is one of the ones reviewed), and I actually questioned him on his recommendation of including a photo.
My understanding is that HR do not want photos in resumes, because it opens the possibility of discrimination on the basis of race, age, gender, attractiveness etc. Many companies have a policy of rejecting any resumes with a photo for this reason - they do not want the risk of being accused of discrimination, lawsuits etc.
David's view was that employers are going to see what you look like in an interview anyway, so if there is going to be discrimination then leaving your photo out of a portfolio won't help. He also said that your portfolio is about selling yourself, so a photo is important.
Ian Fenn also weighed in on the discussion - he said that in all the interviews he had completed with hiring managers, no-one had mentioned photos as a problem. He also pointed out that most hiring mangers will check LinkedIn profiles (most of which include a photo), and that many do an initial filter based on the resume alone (portfolios and LinkedIn profiles come later). I found it strange that the same people who will reject a photo in a resume would accept one in a portfolio, but I guess it is a case of technology and practices moving faster than company policy...
In my own case, I felt uncomfortable including my photo, but I did also see the benefits. So in the end I opted for a compromise - I included a photo which does not clearly show my face, but which I hope still says something about me.