They give the wrong impression of what Libertarians believe.
Abortion is a sensitive issue on which reasonable people can differ.
Some people believe a fetus is a human being who is entitled to the same right to life that will apply after he's born. Other people see the fetus differently, and believe that women have a right to decide for themselves whether to carry it to birth. We aren't going to change either group by spouting contrary slogans at them. This will only identify us as opponents to be disregarded on all issues. In my view, an uncompromising Libertarian position on abortion doesn't have to offend anyone.