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Ultimately, the client is paying you for SEO advice, you are providing advice and they are not acting upon it. Explain to them that whilst non of these issues are massive factors, to be able to provide the best possible advice you need to remove the common errors and be working from a solid standpoint. I like the quality metric - if a site has these daft problems, then it's quality is lower and it's chance of hitting a threshold for a problem is more likely. If the duplication of pages / titles becomes rampant, then we get closer to a panda type problem. Simply put, to rank as well as possible, we do everything we can and taking care of the smaller details is an important part of this job so you can do your job properly knowing you are not being held back by simple, easy to fix issues.

A desktop crawler will do most of that, too - Screaming Frog is a great option, but it's a paid tool over 500 pages (I think). I wrote a post last year comparing it and Xenu, another crawler: http://www.seomoz.org/blog/crawler-faceoff-xenu-vs-screaming-frog