milosevic - 9:03 am on Sep 1, 2010 (gmt 0)
Imagine you have a beautiful garden (your website), with lovely flowers and a pristine lawn (good content). But there's an old car restoration project with an oil leak parked on it and a bunch of old tyres and broken car parts in one corner (low quality content).
You have your new fiance's parents who are very well to do (Google) coming to visit and it's important to make a good impression. You would move/hide the car and try to clean up the rest of the mess (no-index). Because you know that they would only see the old car and parts, not the flowers etc, and remember you by that.
But Google isn't like your fiance's parents. It won't judge you that you're no good because you aren't perfect. Google will see the pretty garden and it won't disregard that just because of some oil stains and junk in one corner.
If pages/content are no good, they're effectively ignored (not indexed or classed as supplemental results). imo you're much more likely to harm your site by mass-no indexing, unless the pages are duplicates or doing things that are black hat or spam/seriously off topic (ie viagra spam).