Are there any specific advantages or disadvantages in generating an XML based sitemap in place of a simple txt based sitemap (list of URLs)?  I realize that in the XML format I can set priority and last-modified date, but it is not clear what benefit, if any, in terms of SEO, etc, this provides.  (For instance, what benefit would there be to setting a low priority, other than possible reducing robot crawl load?)  Explicitly stating this information also increases the probability of a mistake -- some modification date not getting updated correctly, etc.  
Are there specific advantages or disadvantages of an XML sitemap over a TXT based sitemap?
seo;sitemap;xml sitemap
There is no fundamental disadvantage to text-only sitemaps aside from what you've touched on.  Setting the priority and last-modified date can affect the crawl frequency and on a large site with both frequently and infrequently changing content that can be very important.XML sitemaps can also carry media information (e.g. video sitemap) and if you have that kind of site that may be important to you.But if you have a smaller site with mostly set/static content there is probably very little functional difference between text and XML sitemaps.Explicitly stating this information also increases the probability of a mistake -- some modification date not getting updated correctlyNo more of a risk that somehow outputting the wrong URLs in the text version