Normally arguments about definitions don't matter, since it's the meaning behind the words that counts. However, in this case, common lines of attack include saying that atheism/agnosticism are religions just like believing in God or Zeus so neither one is more correct or has more evidence than the other. Another related one is that not believing in God takes just as much faith as believing in God, so therefore both sides are equal in some way. Neither of these is the case, so here, the definitions are important.