Well, the 5400's are adequate for dvr usage, and some of the 5400's with higher platter densities than the 7200's can be pretty fast. At least for basic recording and playback.
When you throw in database stuff (like the new smart search), maintaining to-do lists, vod, mrv, yada yada yada...you need all the response time and throughput you can get.
The 5400 does often throw less heat and less noise.
I think the expectations of the platform have changed significantly. When the directv dvr line first came around directv was co-selling tivo's and they probably envisioned the directv dvr's as a lower end product that would be inexpensive to make and maintain, and compete with the cable company dvr's which are still pretty low end. Then use the tivo product for people wanting more. Then of course they had to piss each other off and hold their breath a long time before (maybe) doing business with each other again. In the meanwhile, the developers at directv had to make do with what they had deployed and try to feature match the higher end dvr products.
Quite a balancing act. I didnt like a lot of the growing pains, but they've done a lot.