Lunt's article starts off by stating some of the benefits of focus groups, which include, getting people to engage on a level where you know what they mean, but also 'how they understand'. That position is followed by Hoijer's point that “the obvious and well-documented effect of group pressure raises too many problems to permit taking the group discussion as a valid basis for research.” I think of my own experience with focus groups, and I can agree to some extent with both positions.
Although random sampling wouldn't, or better yet, hasn't worked well for me in terms of engaging - sitting with a group of people I know and feel safe with, and perhaps who know and feel safe with me, has allowed for results that are honest and useful. Conversely, it is also correct that adhoc group placements are uncomfortable for most, and the group dynamic, as has happened with me on several occasions, will encourage an unequal and perhaps not very forthcoming exchange of ideas and suggestions.
Are focus groups useful, we know they are...are they also problematic because of the pressures raised through group dynamics? Also true!. I guess it boils down to what is being researched and/or observed, and what outcomes are being sought!
Lunt, P. and Livingstone, S. (1996). Rethinking the focus group in media and communications research. Journal of Communication, 46(2), 79-98.