Benefits: default themes are the only ones used for fallback when the active theme has an error. They also have seen more use than other themes, so problems are less likely.

My own use: keep the default for fallback, but use something I like better (I use one I wrote or one I’ve used for years).

Great starting points and perfect for testing.

(1) If I build a 12-page website by adding a non-default theme (like Page Builder Framework) plus a site-builder like SiteOrigin\PageBuilder … but I keep the default themes for backup/testing, does that bloat my site in terms of too much code or front-end size … such that it will slow my TTFB loading/rendering … and thus hinder my SEO and search rankings?
(2) Is there an easy way to know if a theme is based on a framework like Headway or Thesis … to be able to keep my changes in a child-theme to preserve them against theme or core code updates?
(3) How do you all pick a theme, with so many options? i.e., besides trying to compare reviews and author support, do you just assess how many style elements you would keep versus change?

@ Jan: for questions like mine here [essentially, trying to learn a bit before starting to build], should I be posting to the “everything else” / misc. forum instead of “FixingWP”? Just curious, as I want to ‘ask the right way’.

2) Read the theme description. You should probably always use a child theme if your site has custom stuff. Then when you want to switch themes, you have all your custom stuff in one place and can clone the child to work with the new parent theme. Or don’t do anything custom and you would be able to switch themes easily at any time.

3) Themes are all about presentation, so get one that looks close to what you want and tweak from there. Or get one that is really flexible and use it for everything. (I don’t look at reviews or support at all, but then I’m a programmer so that isn’t important to me.)