In most cases thermal compound does not last forever (especially the stock stuff). It dries out and does conduct the heat the way it should. You can also run into a issue that i had, the compound starts to dry out and you have to move video card out of the computer for some reason and all of sudden your GPU is over heating at idle and the fans sound like a jet engine taking off. this is because the thermal compound "broke" connection when the card was flexed even a little and because the paste is no longer pliable you do not have a sufficient connection between the GPU and the heat sink.
So i made this video