Whether someone is likely to have JS disabled or not depends entirely on the target audience. Even so, within each target audience there are clicks of people that have JS disabled, and others who don't. Only your own research would be able to determine what the percentage is for your own site.
I must then decide: client site includes three pages sharing one external js and also sharing three rather long "<ul><li><a href..." sets which the js could writeln out for each of the three pages. Does explicit xhtml coding in each page (with resulting bandwidth but quicker interpretation) outweigh js (smaller bandwidth but more interpretation)? I'll have to decide and experiment.
If you are able to answer "yes" to at least one of the two questions -- go ahead and use it.
Yes, I KNOW the answer to the second question is a firm affirmative, and the first question, in my case, is being rather weakly dealt with, and is part of the reason I started this thread. Right now, all I have on each page is a link to a "site directory". The previously mentioned "<ul><li><a>" group is a navigation <div> with links that prettify by writing status area...lots of bandwidth to put on each and every page explicitly, and lots of maintenance should a page title be added to the nav group.