Thank you both for your thoughtful responses.
The nature of the business I'm in involves documents going through a number of hands in a rather uncontrolled environment. For instance, for any given five or six authors on a proposal, three may be working in 2007, the others in 2010, or perhaps one or two in 2013, all on their personal computers out in who knows what hinterlands here or abroad. Thus, there are practical limits to what we can do to make Word set-up on them all the same. I have only met one person in a major corporation of our type so far, who successfully implemented a style-limited template process, using a custom toolbar in a self-extracting-installation template in W2003, and his proposal authors weren't constantly changing.
Although we have a template which goes out to be used for authoring, one carefully created with Ms. Kelly's expert directions and other such expert advice, we inevitably deal with documents returning, at various iterations of the extended process, with text and tables pasted directly from a wide range of sources by folks who simply don't know better. When we can, of course, we move everything cleanly to a new template-based document, but that is not always possible. The erosion to which I referred, is acceptance of bad practices and their effects continuing until too late in the process to rectify. If one author, for instance, has automatic style-updating checked and doesn't notice it's effects...
As to stability, I refer to my repeated experience of receiving documents in which some kind of corruption has been introduced through such uninformed assembling of documents (we do not use master documents in our process, only template-based ones), which may manifest itself in the form of auto-numbering problems, extended tables carrying a host of persisting "ghost styles" with them, and similar things, which can simply make fixing them a long process in a multi-hundred-page document when no time is available for it. I am aware that a long style list is not a problem per se.
Yesterday I had to send out a 350-page "finished" document into which eight older documents had been inserted, each with its own TOC and styles list, some of which styles had the same name but different font and paragraph characteristics, along with numerous instances of Normal text with different formatting, etc. Frankly, I was worried about what might happen each time it was opened.
So, I was just probing, in my post, for whatever consensus there might be on the consequences of bad practices. I will continue to make my case here against them.