Yes, they can. See Brad Geddes' blog post at http://www.ewhisper.net/blog/low-quality-keywords-can-lower-other-keywords-quality-scores/ where he has some answers from Google that explain how the account quality affects the QS of all keywords in the account. They may not make a huge difference, but there do have some effect.
So using that logic my keywords with high quality score should have a positive impact on the other keywords in my account, right? Doesn't make sense to me. From Brad's post, I'd pay attention this: Now closer to the bottom of the page... All that said...I've worked with some pretty large (and small) accounts and I've never seen "low quality" keywords have a measurable impact on the other aspects of the account. Maybe they do, but if so it's been so miniscule I haven't noticed it even when I've been looking. If you jump over to Brad's presentation and click on section 8 (Account Quality Score) you'll see: So following that example lets say I have two keywords - one with a "great" quality score and one with a "poor" quality score. Now lets assume the real quality score is a numeric rating between 1 and 5, with 5 being the best. My "great" keyword gets a 5 and my "poor" keyword gets a 1. Combine 5 and 1 and you have a 6. If I remove the "poor" (1) keyword I'm then just left my "great" keyword - 5 + 0 = 5. I read pretty much everything Brad posts, including all the stuff linked to in this post. My only issue with the presentation and the information it contains is that it, at least from what I've seen/read, has never been verified as accurate by anyone from Google, and, Brad doesn't site any sources as to where he obtained this info. If everything in his presentation is 100% correct why hasn't Google added some of that info the AdWords FAQ/Help? Why have all the Google account reps I've worked with given me (and others I know) conflicting information? Sorry to swing this post somewhat off topic. I guess my point in all of that was I personally don't think "bad" keywords have enough of impact on other elements of the account to matter. That statement is based on what I've personally seen over the years using AdWords. Hopefully at some point in the not to distant future Google will add some official documentation to their FAQ about this. Seems like a pretty simple thing to address don't you think?
I thought I'd jump in here and clarify a few things. First off, the information comes from AdWords seminar presentations which are checked by Google (as the seminars are also approved by Google). http://www.google.com/awseminars The account quality score is not a huge factor upon quality score, however, it is a factor (and one that's controllable/measurable, unlike the other 100+ factors that only matter at the time of the ad serving). The account quality score is not going to make a huge difference in your account; however, in highly competitive industries, every little thing can help. The other item its used for is setting the initial minimum bids. If you are using keywords for which there is little history, Google often has high min bids. A high account QS (along with the changes they are rolling out soon) are suppose to help the initial bids to be more reasonable for when there's not predictive information. Hope that helps a little.