I think another interesting factor is that if you go far enough out, the points are very consistently valued.

California Zephyr, Chicago to Sacramento (two zones) in January:

$145 for coach or 8000 pts, $0.018 per pt
$407 for roomette or 20000 pts, $0.020 per pt
$597 for bedroom or 30000 pts, $0.020 per pt

But in real life, bedrooms inevitably sell out first, then roommettes, then coach seats. And first in this case may mean 3 months out. Also roomettes and bedrooms prices go way, way up when they get into the last buckets. Maybe it's proportionally the same as a coach seat, but $600 going up 100% is much harder on the wallet.

For instance, if I try that same search in September, they still have those coach seats at that price, but the others are much dearer:

$779 for roomette, $0.038 per pt
$1262 for bedroom, $0.042 per pt

Given all that, I'm not sure what the conclusion is. Just that if you're booking far in advance, the sleeping cabins may not be the automatic best user for you points that they are most of the time.