‘Jesus therefore said, “Leave her alone, that she may keep it for the day of my burial. You always have the poor with you, but you do not always have me with you'.
This is a brief summary of what Jesus actually said. Mark's clarification makes it easier to understand. John knows that his readers have the background of the tradition behind Mark's Gospel, but because he wants to emphasise the contrast between Mary and Judas rather than the significance of the anointing, he keeps the speech as short as possible.
Interestingly Jesus' comment both approves of their general attitude while at the same time gently rebuking their lack of insight. ‘Let her keep it for the day of my burial'. Mark says ‘she has done what she could, she has anointed my body beforehand for burial'. What Jesus therefore intended them to understand was that this moment must not be spoiled by arguments. As with the High Priest earlier (John 11:49-51) her act was unconsciously a prophetic action (although possibly subconsciously it held within it a hint of prophecy - it is such as Mary who sometimes have a presentiment of doom, and she would know of His earlier teaching about His future suffering at the hands of the Jewish leaders, and she would be well aware of the threats that were going around). She had by it anointed Him for His burial in advance. So He means ‘let her keep what she has done for the day of my burial' - when that day came and He was buried unanointed, she would say to herself ‘I anointed Him in readiness for this' and be comforted. What seemed like extravagance would be seen to have been a necessity.
We need to recognise that the writers of the Gospels do not try to tell us everything that Jesus said to its fullest extent. They do not see themselves as simply doing an interview and repeating everything word for word. Rather they are men putting over a message. Possibly what Jesus actually said in full was, ‘Leave her alone. Why do you trouble her? She has done a beautiful thing for me, let her keep it for the day of my burial. You always have the poor with you, and whenever you are willing you can do good to them, but you do not always have me with you. She has done what she could. She has anointed my body beforehand for burying. And truly I say to you, wherever the Good News is preached in the whole world, what she has done will be described in memory of her'.