I think this is useful in "Sleep and hibernate" mode, but the value of always hibernating seems questionable to me. Having had it in always-hibernate mode for a week or two now, I ended up switching to sleep-and-hibernate.
I have not timed it, but I would estimate that shutting down and powering back up, especially with 10.4's extensive caching, is just as fast as going to hibernate and back, at least on my 2 GB RAM, 1.5 GHz PowerBook G4. Plus shutting down has the advantage of giving you a clean slate to work with at the next power up and you have the option of a quick sleep/wake when needed. If you set your powerbook to always hibernate, then you lose that quick sleep/wake capability.
The as-long-as-full-startup restore from hibernate mode takes away most, if not all, of the advantage of sleep mode, and hibernate mode, like sleep mode, can't be trusted - you should save open documents before using it. Given that they are saved, there is little advantage to hibernate mode versus shutting down, since after a normal powerdown/startup you can just open the saved documents and be where you left off. Moreover, shutting down reminds you to save unsaved documents, so it is inherently safer. If hibernate mode were nearly as fast as normal sleep, then it would hold an advantage over sleep, but since it is so slow, its value as as sleep replacement, as opposed to a supplement, is highly questionable.
As a supplement, however, for when your powerbook/ibook accidentally loses power while asleep... that can be useful. It may prove to save ones bacon some day. So set it for "Sleep and hiberate" mode, and use shutdown, not hibernate mode, when you want to minimize power drain.