The resons are generally pretty simple, and a personal choice
For myself, and 'headless' photography, I have too many computer-savvy and inquiistive kids around here to take on any more risk than I already do in the areas of 'exposure', and, I have quite a lot of facial hair that totally blows the look. (Its especially a bummer because I love lipstick, and you just can't see it through all that!)
** I considered making up some kind of "All the old family photos had heads cut off so I thought that is the way it is supposed to be" but I thought better of it! :)