I've just done so, and I intend on keeping Web History turned off. I simply do not derive enough benefit from having this data around for myself (when's the last time you benefited from Web History), and unlike other permanent data such as G+ posts, comments etc I do not self-censor searches that I would like to conduct, so I find it very plausible that someone with more processing power than me could find out more about me than I intend.
I was just about to and then realised that I had turned it off a while ago. The fact that I had completely forgotten it shows that I don't need this data.
However, it's also not something I'm hugely concerned about; as others have said the main danger is from some unscrupulous individual gaining access to your account. In terms of Google having access to my data, I would imagine my data will never be seen by human eyes. Having said that, one does need to remember the inherent permanence of data on the internet, as Facebook's timeline has shown sometimes your data can surface in unexpected ways.
