I don't follow Google's algorithms too closely, so someone correct me if I'm wrong on any of this, but I always enjoy talking about it. As far as I know, the words in meta descriptions aren't used at all for ranking purposes, and never have been. But they have a ton of value for user marketing purposes, since that text actually appears to the user at Google. So a finely crafted meta description can be what makes the difference in a user clicking on your site versus another in the search result pages at Google. The same goes for inclusion of keywords in path structure, which get bolded if they match the user's query, marketing to the user in yet another way (even if negligible for actual ranking). You can build a site that performs well with search engines and completely ignore the meta description. But you'll get more people clicking from the search results to your site if you use them well.
By that token, the meta description's value can probably be seen as very important from a traditional marketing perspective. But I'm not a copy writer and often don't know how to compel one with words in any special way. So if I (as the developer) don't have the responsibility of writing content, I might prefer to have Google auto-generate one for me rather than trying to auto-generate one at the site. (i.e. omitting the meta description or leaving it blank). Though if there is already a summary or worthwhile sub-head field included with the content, they might be perfect for the meta description. But I don't think there's any reason to be afraid of Google's auto-generated description, unless you really have good human-written content for it.
The <title> tags are even more valuable for user-marketing. But unlike meta descriptions they have always carried a lot of weight for ranking. Though if my experience is correct, Google is being even more picky about what it considers a good title tag than it used to. If Google thinks your <title> tag is trying to speak more to keywords than to the user, it's value is reduced. This has always been the case, but I think it's become more so. What always seems to work well is a <title> that forms a focused and intelligible phrase (or short sentence) that reads well to the user and uses a target keyword (or keywords if they read together) in a natural way.
These details are the things that we are responsible for and are able to focus on. But ultimately, even if you do everything right on your site, 80% of search performance still has to do with what's happening elsewhere and things that may be out of our hands. Most importantly: what quality sites are linking to you and why they are linking to you. But also: how good and unique the site is (relative to others) and how long it's been around… things that tend to correlate with link quality.