You can use Dynamic Keyword Insertion (as Flip says), but this won't count towards your Quality Score. If at all possible, you want to put your keywords into very small adgroups, with the keyword in the title (or, if you use DKI, as the alternate text). This tells Adwords that your advert is relevant, and so your QS will improve, giving you cheaper clicks.
My understanding is that when calculating your Quality Score, the default term is used. I've never seen my QS improve directly from using DKI, though as (I think) you say, it can have a very positive impact on your clickthrough rate, and improve your QS that way... Don't see how it can hurt your ROI - can you elaborate?