I'm assuming you're talking about the dreaded "mere" dayhikers who start at a car in a parking lot and end at a car in a parking lot THE VERY SAME DAY and are therefore out for a "quick walk". My elitism tells me to avoid these people when I'm out on a long backpacking trip and for good reason. This has nothing to do with predation or encountering questionable people unless you consider dayhikers to be questionable in that they start at a car and end at a car etc etc etc.
I have found thru experience that while encountering dayhikers is rarely dangerous, they can be detrimental for a backpacker who wants to keep his head in the game, so I often try to avoid conversations with them. Remember, you're out for 3 weeks and they aren't---their head's are already back at the car and at home but your head has to stay out and stay in the game. What the heck am I talking about?
First off, many of them think they know more about the trails they are hiking than anyone else and seem to get out of sorts when you ask them detailed questions of the area. Second, some of them get perturbed to see a tent and someone camping on "their" favorite hiking trail, like you're illegally camping in a city park.
Third and possibly the worse---many dayhikers and dayhiking groups cannot conceive of actually stretching out a day's hike into two days and spending the night. So, you often encounter these people fresh from traffic, a car and the road, and anxious, excessively anxious, to get back to their cars and a restaurant meal on the way home.
So, the question is, should a solo woman backpacker avoid dayhikers? Yes. by all means.
END OF ELITIST RANT #74