And used out of context
While Jesus was in Bethany in the home of Simon the Leper, a woman came to him with an alabaster jar of very expensive perfume, which she poured on his head as he was reclining at the table.
When the disciples saw this, they were indignant. “Why this waste?” they asked. “This perfume could have been sold at a high price and the money given to the poor.”
Aware of this, Jesus said to them, “Why are you bothering this woman? She has done a beautiful thing to me. The poor you will always have with you,[a] but you will not always have me. When she poured this perfume on my body, she did it to prepare me for burial. Truly I tell you, wherever this gospel is preached throughout the world, what she has done will also be told, in memory of her.”
Matthew 26:
http://www.biblegateway.com/passage/?search=Matthew+26&version=NIV
Thus it is a rebuke of using the poor as a weapon against learning. Christ was on earth to teach certain lessons, one was to take care of the poor, but the fact that the woman wanted to honor Christ was itself good, and to use the poor as a weapon against her was wrong. Remember this is after the raising of Lazarus (which is NOT reported in Mathews), the biggest and last of Jesus's miracles while on earth. The woman wanted to honor him for what he had done, to refuse her would have meant to insult her and that was wrong.
This is made clearer in John:
Six days before the Passover, Jesus came to Bethany, where Lazarus lived, whom Jesus had raised from the dead. Here a dinner was given in Jesus’ honor. Martha served, while Lazarus was among those reclining at the table with him. Then Mary took about a pint of pure nard, an expensive perfume; she poured it on Jesus’ feet and wiped his feet with her hair. And the house was filled with the fragrance of the perfume.
But one of his disciples, Judas Iscariot, who was later to betray him, objected, “Why wasn’t this perfume sold and the money given to the poor? It was worth a year’s wages.” He did not say this because he cared about the poor but because he was a thief; as keeper of the money bag, he used to help himself to what was put into it.
“Leave her alone,” Jesus replied. “It was intended that she should save this perfume for the day of my burial. You will always have the poor among you,but you will not always have me.”
Meanwhile a large crowd of Jews found out that Jesus was there and came, not only because of him but also to see Lazarus, whom he had raised from the dead. So the chief priests made plans to kill Lazarus as well, 11 for on account of him many of the Jews were going over to Jesus and believing in him.
John 12:
http://www.biblegateway.com/passage/?search=John+12&version=NIV