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.