I think the premise here is the same with regards to HTPCs as other hardware capabilities, like processor speed, memory, and graphics card. If you don't need a high-performance HTPC, you can do without an SSD, too. But an HDD makes more sense for the HTPC for other reasons, too:
- The machine is rarely restarted. Virtually no impact on the user.
- The apps are pretty small overall, so they are quick to load. Very small impact on the user.
- Ripping DVDs and blu-rays are limited by physical media speed, not HDD speed. Virtually no impact on the user.
- Processing video uses sequential reads/writes, which HDDs handle fine. And, they can be automated to run when the user isn't making use of the machine. Virtually no impact on the user.
- Media is the most important part of the HTPC, and that's all on the HDD/LAN storage/Internet anyway. Virtually no impact on the user.
Now, this is the most generic of usage scenarios for the HTPC. Some will fully shut down their HTPCs. Some will play games or run "real apps" on their HTPCs. Some will set up SSD storage. Some will process video in real-time and wait for it to finish. There are a lot of scenarios you can imagine where an SSD would immediately impact a user.
But you can also imagine the scenario where it doesn't. So I think the HDD-only case is pretty valid.
Now, that being said, I'm installing an SSD in my HTPC as soon as I get around to replacing my 80GB X25-M in my main machine with a 240GB Agility 3!