According to "proper speech" in Buddhism's eight-fold path, is cursing allowed if it's necessary to get the point across? One of the points of Buddhism the eight fold lists "proper speech," but does this propriety in speech include the ability to insult others or express yourself strongly through the use of curse words/ cursing if it's necessary to get the point across to someone who wouldn't get the point otherwise? Why/ why not?
One of the Five Precepts (pañca-sila) - the basic training rules that is observed by all practicing Buddhist lay men and women -  is to follow the precept of abstaining from wrong speech. Wrong speech involves a great many things apart from uttering falsehoods. It includes insulting speech, malicious speech, even gossip.  This in positive terms is the practice of Right Speech.  It is said that "If you can't control your mouth, there's no way you can hope to control your mind.” This is why right speech is so important in day-to-day practice. 

Cursing if it's necessary to get the point across to someone can be termed as harsh speech (spoken with the intent of hurting another person's feelings). The important point is that you were mindful of this slip. I do not see it as a transgression. It is not a sin as that of a transgression against a god, as in other faiths. This is only a trifling unwholesome act done by one who strives to practice the Dhamma, as opposed to the same act by another. What I said above is in keeping with the scriptures. 


  “Suppose that a man were to drop a lump of salt into a small amount of water in a cup. What do you think? Would the water in the cup become salty because of the lump of salt, and unfit to drink?”
  “Yes, lord ….”
  “Now suppose that a man were to drop a lump of salt into the River Ganges. What do you think? Would the water in the River Ganges become salty because of the lump of salt, and unfit to drink?”
  “No, lord ….”
  “In the same way, there is the case where a trifling evil act done by one individual [the first] takes him to hell; and there is the case where the very same sort of trifling act done by the other individual is experienced in the here &amp; now, and for the most part barely appears for a moment.”
  — AN 3:99