Putting PCM tracks on dvd's isn't really practical unless your video has a fairly short run time and you don't need your peak video bitrate to go all that high. With the audio at 1536 kbps your video would have to be encoded so that it doesn't go over 8.3 mbps or something like that. For some things that won't be a problem, but for the Star Wars films it may not work all that well because they are over 2 hours and have lots of fast-moving action scenes. It could be done, but using an AC3 track at 448 kbps will give you more breathing room and there won't be too much of a drop in sound quality compared to the source.
A bitrate of 448 for stereo is quite a lot compared to using 448 for 5.1, since the same bandwidth has to be spread out over more channels. I've noticed at hifi stores, where the sound quality is greater than what I have at home, that playing 5.1 AC3, while quite good, isn't really that much of an improvement over my own system in terms of fidelity (although dynamics are another matter due to more robust amplification). But playing music with uncompressed PCM is another matter, whether it's Vivaldi or the Alien soundtrack cd or whatever. The benefits of higher bitrates become more apparent as the quality of your equipment improves.
So I suppose I would say that using the highest practical bitrate is always advisable, but not necessarily the highest possible in all cases. If your player can swing AC3 at 640 kbps (out of dvd spec but I think some can do it), then that, of course, would be the best compromise between quality and bitrate; but that won't always be possible.
On another note: since this is the LFE thread--I was wondering if anyone who downloaded them, either in stereo or 5.1, would care to comment on their experiences with those soundtracks. How well did the bass integrate into the movie--did it enhance the movie, or detract from it, or not call particular attention to itself? How was the surround sound if you have it? I'm interested to hear folks' impressions, whether general or specific.