Benefits of using a base theme? In building a new theme (for a bespoke build, rather than distribution) is it essential to start with a base theme? If I look at the Zen theme, for example, I would scrap most of the HTML and CSS in order to start with a blank-er canvass than that theme provides. Are there still many benefits to using a theme like Zen in that case? 

Update
In an attempt to clarify, this is for a professional project, and in terms of HTML/CSS we'd usually start from as little as possible; normalize.css, modernizr and not much more. Most themes seem too 'opinionated', there's too much we'd have to re-style or remove to the point where it would slow development down.
If you're not doing a traditional 3 column with header and footer site layout, then no - you probably won't get much out of something like Zen.

That said, you can still leverage the awesome Zen Grids gem with SASS and get the benefits Zen's grid system without any of the cruft from the base theme.

Anyone who has themed enough Drupal sites understands that you're going to be dealing with machine-generated HTML at some point (either from Views, or some other contrib module). You could spend a ton of time overriding all the theme functions to get the exact HTML you need so you can apply the col-xx classes your grid system of choice needs, or you can use the classes that already exist on the pre-existing markup and make them compatible with your grid with Zen Grid's mixins.

I like to say that Zen Grids acknowledges the reality of Drupal theming and comes up with a system that works with Drupal as opposed to against it.