Here are some considerations for doing portraits with manual focus or autofocus. Im using Nikon to illustrate.
1. Some autofocus systems are better than others. Even with a superior portrait lens on a superior camera with a superior autofocus system (EXPEED 4) I am often surprised at how far the camera can move without auto refocusing. Surely this fear is absurd but I do force it to refocus when this happens or else I move to manual focus. My point is, the more control the photographer exerts, the faster the shooter comes to understand successes and failures.
2. The contrast autofocus of Nikons Live View is suited to focusing on faces and other fairly stationary subjects. Live View works across the frame but you can pick the focus point. Can Live View refocus in response to a sitters movements? Dont know but worth investigating with a sitter.
3. If you are using a high-pixel-count camera conceived with attention to studio work like the D810, for example, Nikon recommends a tripod and Live View.
http://nps.nikonimaging.com/technical_solutions/d800_d800e_tips/shooting_techniques_static_subjects/. I would also consider a focus rail. This configuration allows for previewing the shot, possibly magnified or on a large monitor. These techniques make photographers efforts in manual mode more competitive with autofocus. Likewise, these three techniques work well with autofocus.
4. The Live View image comes from the sensor and so works around any viewfinder issues mentioned in the discussion here. Changes made during previewing are sent to the live view. Similarly, the need to change settings is more likely to show up. You can change the aperture of a Live View preview to manage depth of field and point of focus.
5. Some autofocus lenses have short throws their focus rings have little travel distance. My 105mm G has an effective focus-ring rotation of no more than 90 degrees under its manual setting. In contrast, some Zeiss manual lenses have much longer throws, which allows for more manual precision.
6. Some manual-focus lenses have narrow focus rings; Nikon has been known to update lenses to wider rings and these are the norm.
Working with manual focus sets the table for more advanced focusing techniques involving depth of field and focal plane. For example, Nikon has an interesting full-body portrait taken with their PC-E 85mm lens; the shifted focal plane pushes attention to the face. Similarly, the Nikkor 58mm 1.4 G offers a progressive intensification of DoF, according to
http://nikkor.com/technology/02.html; this is independent of how focus is achieved.