HTPC is not just a video playback device. (those you can get for about $100, I use the WDTV Live HD
)
HTPC is also about media storage, media sharing, recording broadcasts, converting formats. Many Audio video nuts also enjoy fine tuning their Video and Audio playback. Which requires a beefy CPU for 1080P and high bit audio.
(FYI, 1080P movies you download, is compressed. BDROMs are 25GB, 1080P movies in MKV are about 8GB, most people will not be able to tell the difference some will complain though).
That does not even include Flash playback (not happening on the ATOM I had). Nor does it include gaming which many people find much more enjoyable in a comfy chair with a 10' UI like console gamers. Because of the generally lower resolution (1368x768 for 720P TVs) an even more powerful CPU is needed to keep the GPU from starving.
All that said, no it is not necessary. My HTPC is a unlocked sempron 140 running on a 890GX with IGP. Handles media serving duties. It will run flash okay, Java apps are another story. That can completely bog down the system and make even streaming impossible.
But as a 2nd PC in the living room, it is handy when the kids are on the house PC, I can pop on the HTPC real quick to check stuff.
I pulled the tuner from my HTPC, so it is really just a media playback/server now. Depending on how H67 turns out and if they release cheaper CPUs with 12EU graphics I might go that route for HTPC