If you have DirectTV you need nothing. But, being able to get digital cable and the premium channels directly into a TV without additional fees for an ugly box is pretty nice.
I still struggle with rear projection units that use a light bulb you will have to replace once a year at $300.00 a pop. You get a 50" set, it costs you 2 grand or more and then you have to maintain it regularly... and it's 18 inches deep. At 18" or 10" it's still to deep for me. I would rather get a 10 foot screen, mount it an inch off the wall and use a Sanyo PLV-Z2 and enjoy a real home theater experience.