Word is an excellent word processor. InDesign is an excellent pagelayout application.
I’ve seen great stuff come of Word and I’ve seen absolute crap come out of InDesign. It’s not about the applications, it’s about the proper use of the right tools for the right job.
In this case, there is a lot of crap coming out of Word and very little coming out of InDesign.
Expanding on Bob Levine's succinct comments and as someone who uses InDesign for virtually all text-based document work, except when I am required to use Word (such as for ISO standards documents)…
In terms of any perceivable objective quality difference between the resultant PDF files of legal documents produced in Microsoft Word versus InDesign, differences would most likely be due more to the training and discipline of those keying and formatting the documents than the tools themselves. There is virtually nothing in the requirements for typical legal documents that requires the sophistication and capabilities that InDesign provides over Word.
Word has a relatively gentle learning curve although accessing certain advanced features may be an exercise in frustration. InDesign has a very steep learning curve, but once one has gotten past that learning curve and maintains discipline in use of InDesign with proper use of Paragraph, Character, Table, and Object styles as well as named colors, accessing advanced features is relatively easy.
However, what I will observe is that InDesign users who have mastered the product are more likely to be conscious of use of typography, formatting, and the general “look” of their documents simply because of that emphasis in Adobe products and the propensity of such persons to be care about output quality. And those InDesign users would also be much more likely to produce gorgeous legal documents using Word because they would look for and use comparable, albeit somewhat hidden, formatting and style definitions in Word and (such as pair kerning, ligatures, etc.)
All that having been said, in reality, if you are in a law practice and you need to often bring in “temps” to work on legal documents, requiring proficiency in InDesign might be very limiting in terms of finding suitable personnel and/or you may have a disaster on your hand when someone not trained in InDesign tries to edit existing or especially create new legal documents in InDesign.
Of course, if you want your law practice to produce some great marketing / promo material, then InDesign is the more appropriate tool.
What an elegant and knowledgeable reply!
You have a much more limited sampling than I do. I can assure you of that.
I think you should also consider to use Adobe FrameMaker for legal documents as it has strong capabilities in dividing content and formatting and has some formatting neither InDesign nor Word have like fixed numbered tabulators (contrary position relative in InDesign), inline paragraphs (a paragraph which produces no new line in the beginning), marginal text without an anchored frame, very complicated numberings are possible etc. But it is difficult to learn and only available on the Windows side at the moment.