The Porter is on one level a naturalistic portrait of working class living conditions. What could make us feel a certain sympathy for him?act 2 scene 3
Working class citizens in this time period were not given much consideration. The lower class were considered equal with animals...especially if they were from Ireland or Scotland. The English opinion of people from these countries was (and some may say still is) one of contempt and loathing.
We may feel sympathy for the Porter not only because he is a member of the working class, but also because it is his job--regardless of the state of his health or how much sleep he has had--to answer the door as well as perform any other duties assigned to him. One may also feel sorry for him because he is obviously inebriated and carries on in vulgarities regarding the effects of drink on his ability to perform his duties (double meanings are everywhere) which were common and expected of the lower classes. He is pretending to be the gatekeeper to Hell. Living in such a place where evil such as murder is so flippantly committed is enough to feel sympathy for any and all of the servants who must endure it.
The Porter plays his "gatekeeper" role well, and Shakespeare's audiences would have immediately recognized that he was portraying a part from the Medieval Morality Plays--the gatekeeper of hell who admits Christ to Limbo in the ancient legend of the "Harrowing of Hell".
It is clear that we are to see Macbeth's castle as Hell and that Macbeth has lost his soul. Even so, the Porter is much needed comic relief.
I just want to add something to elaborate on the previous answer. Although he came later than Shakespeare, Samuel Johnson gave a good indication of the way poor people, especially the Scots, were considered. In his dictionary, Johnson defines "oats" as "a grain, which in England is generally given to horses, but in Scotland appears to support the people."