It is illegal, at least in the USA, to upconvert DVDs above 480p resolution and output them over a non-copy protected analog output such as component video.
HD DVD/Blu-ray are both HD native sources, and while they are native 1080p signals most of the time, they may only be output at 1080i or lower resolution over component video.
For 1080p (if possible) from your player, or for 1080i of all sources, you must use HDMI.
Why are you NOT using HDMI is really the question? That's what I would just do by default unless there was some obvious reason not to.