The only place a trimmer would really be useful is the unibrow, everything else is too hard to get to or too easy to mess up. There is also a noticeable difference when the hair grows back from shaving than from plucking.
“The hair shaft tapers at the end. ‘When you shave it, you’re crossing the midshaft, so it seems like it’s thicker and more coarse,’ explains Dr. Robin Ashinoff, director of cosmetic dermatology at Hackensack University Medical Center. ‘But it’s not.’” (http://www.newsweek.com/id/78014)
So basically the hair keeps growing, and the middles are thicker than the ends and thus more visible.