Hrm. It only gets quartered once, though, no matter how much it's shared? I bet that this could still be turned to your advantage.
A single 21x21 (or larger) double-engraved room filled with masterwork furniture could cover all your noble needs forever. In fact, why stop at nobles? Just keep expanding and adding beds, and make a shared room for every single dwarf in your fortress, plus your dining room. With solid gold furniture. If you've got a legendary carpenter, I'd bet that all the masterwork beds alone would, in the long run, add up to enough to counterdict the one-fourth-value problem.
(Say that most of the value of your rooms is coming from your masterwork beds. Four dwarves sharing a room would already be enough to counter the 1/4th value effect. Eight would give them double the old value. And that's not counting the constantly-increasing value from constantly-increasing size, or the fact that you can throw masterwork statues or whatever in the room to keep everyone happy. If you ever get the chance to place a piece of artifact furniture, it'd leave you set for life.)
Your nobles would also complain about uppity lessers, probably, but with a valuable enough room it wouldn't matter.
...is there any reason why this wouldn't work? Now I'm tempted to try it.
[ August 23, 2006: Message edited by: Aquillion ]