Many STBs look better over HDMI than component it seems. Mine looks the same either way, but my Maxent 50" plasma does this odd auto brightness thing that adjusts the brightness dynamically over component. Doesn't do it over HDMI.
Besides, virtually all upconverting DVD players only upconvert over HDMI. It would be silly to hook, say, an Oppo 971 up using component when you can use HDMI.
HDMI cables also take up less physical space than component cables, which can be important in some installations.