Mark,
Ross, and others wrote that par 3's could be a bit more difficult than other holes, since the ball started on a tee. I agree, and it could be, as others point out, a test of accuracy, putting, etc.
I also think they are, given no shot relationships strategies, a good place for interesting shots, and concept shots, like the Redan hole, etc. but they don't particularly have to be more difficult in putting.