For whatever it may be worth, here's my corporate experience as the person in charge to proposal templates and DTP at my company.

For proposals, we use a few custom templates. The problem with SP is that the document gets divorced from the template. I have not found a way around this. Also, the writers and others working in the documents, outside our proposal management team, tend (understatement) to paste material straight from old douments, the web, Excel, wherever, with all the attendant styles coming with it. My experience is that heading numbering, bullets, etc. -- autonumbered lists -- get frequently stripped of the their auto feature, or it changes from number to letter or vice-versa, requiring me to reattach and reapply the correct template or, sometimes, individual styles from it. Later today, I will be taking a document, without it's final, empty paragraph, and porting it over to a fresh document based on the relevant template, because the stripping is happening about once an hour. I'll also be replacing stlyes that have accumulated seemingly endless names from character styles being created in paragraphs. Not the first time this has happened.

I have posted here and elsewhere for a solution to this to no avail, given the other restrictions in the way our work goes. For example, it is not practical for us to restrict style changes completely in proposal documents.

Years ago, back in the Word 2003 days, I worked briefly for a fellow who had spent two years working with MS to craft a corporate template that was locked. No styles other than company ones could be used; anything pasted in directly came in a Normal and had to have a style applied, which was done from a custom toolbar. This was only possible, because a self-extracting template was issued to each person working in a given proposal document, which then provided the style activation for the user on his or her machine.