You could consider a RW/SW to be mostly a prestige object... although, in the Peacemaker-type game (that I set aside for awhile when I found out about the JA2DL user-created campaign; killed Deidranna (well, several of them... strange mod) again) with a hefty resource surplus I'd started to construct sphereworlds around stars that were listed as unstable, as a preventative measure. I seem to recall that once you get the "star is about to explode" warning, you don't have enough time to build an SW without refit-chains, and you may not be able to build an RW unless you have sufficient spaceyard ships already assembled somewhere and ready to go... and it would have been *cough* irritating to lose a core system to a preventable incident. SWs/RWs prevent such stellar catastrophes (including artificially-induced ones), and are also immune to planet destroyers.
One note is that they start breathable / optimal, and with 150%/150%/150% values. Thus, they're great for monoliths (if you *need* the resources at that point...). The additional population capacity also means that, eventually, you can get some higher production modifiers than you can with a "mere" breathable-huge. It'll still take you basically forever to fill the world with monoliths, unless you're Temporal in which case it'll take you a fraction less.
[Edited when colonDL turned into, well,
L...]
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-- The thing that goes bump in the night
[This message has been edited by Taqwus (edited 24 July 2001).]