I don't typically watch those videos, except someone on here, I think it was Arfa, did one recently... and it was well-done.
You have to do it right, it has to make sense. In Arfa's case, I forget why he did it, but his voiceover was in between well-edited music sections, and the whole video had a great vibe to it.
Myself, I sometimes throw footage up in the background or scale myself to a smaller window to talk about something when I'm doing an off-bike video, but the footage gives the viewer something better than my ugly mug to look at while I talk. My Vaquero vs Street Glide video comes to mind.
Point is, it CAN be done, but not on a regular basis, and only if done right. Then again, it's YOUR video, so do whatever feels right - don't let others dictate how you do something
-John