If you jump around a lot in a recorded program (or maybe skip ahead over commercials), the hdd will tend to be more responsive to your jumps than a disc recording. It's not a mind-blowing benefit, but certainly a nice perk if you really hammer those skip ahead/back buttons.
Also, if you find yourself in the scenario of wanting to playback something while something else is currently recording (very useful if your recording habits seem to be outstretching your available time to actually watch the stuff), the hdd is a natural performer for such tasks. If not a built-in hdd, the recorder should have dvd-ram support, if this sort of operation is important to you.
I need your sweet love, Rosetta Stone girl!