To me, there is a certain irony that such a widely-celebrated holiday commemorates a Christian missionary, for Christian missionaries are not very politically correct these days. Missionaries are often portrayed as being self-righteous and humorless, and culturally insensitive. In fact, they are often accused of being agents of cultural imperialism. Indeed, missionaries are often blamed for destroying the cultures of the peoples among whom they work. Moreover, it is not only in Western countries that such negative perceptions about missionaries can be found--having lived in China and studied Chinese history, I am well aware of the fact that missionaries have often been viewed in a negative light in that country, especially (due to ideological reasons) by the Chinese Communists.
To be fair, some of these stereotypes about Christian missionaries in the past (and maybe the present) have an element of truth to them. It would be dishonest to deny that missionaries sometimes have behaved in a self-righteous and/or cultural insensitive manner. However, ultimately these negative portrayals of Christian missionaries obscure an important truth--that missionaries have often done a great deal of good for those people to whom they were sent.
Of course, as a Christian, I would say that the greatest good that missionaries have done is to bring the Gospel of Jesus Christ--the greatest of good news--to those who have never heard it. However, even non-Christians, if they were to examine the historical record honestly, would have to acknowledge that Christian missionaries have often been agents of positive change within the cultures in which they worked. For example, in China, missionaries promoted the education and, more generally, the better treatment, of women and girls. Sometimes it was missionaries who brought about the end of horrific religious and/or social customs. St. Patrick himself is a case in point. As this article (see here) from The Independent (Ireland) points out, Patrick was responsible for destroying an ancient pagan religious site in Ireland where human sacrifice was practiced. I doubt too many would argue that this was a evil act of cultural imperialism!
In short, for all their shortcomings as imperfect, sinful human beings, Christian missionaries have often been a force for good in the world. Therefore, it seems highly appropriate to have a day on which to honor one of their number, St. Patrick, the "Apostle of Ireland."
Image of St. Patrick from stpatswichita.org