...why should I have to file a DMCA complaint every time?
Why? Because google isn't smart enough / doesn't care enough to ban the scrapers automatically. So you will have to do SOMETHING. I am guessing that filling out DMCAs, contacting hosts, contacting adsense would be cheaper and faster than hiring writers to rewrite content. And what is to stop them from scraping the content again AFTER you have re-written it?
I notice a lot of people recopying my content on Yahoo answers and then posting a link to their site. Anyone else?
Yes, I think that is somewhat common. It happened to me and I filed off a couple of DMCA requests with google, and within a week those pages were removed from the google index.
Having said that, there are some people who suggest that you DON'T do that IF the yahoo group site is linking back to your site, since having a link from a yahoo domain would have high page rank value.
That might be true; I would check to see whether that yahoo group page (which scraped your content) has lots of good, juicy inbound links, and would also check in analytics to make sure that you are getting good traffic that converts from that yahoo groups page. If so, you MIGHT want to leave it alone...
However, I did notice an increase in rankings and traffic to my page AFTER I filed the DMCA requests against the yahoo groups pages that were all linking to my (single) page. Whether the increase in traffic / rankings is related to filing the DMCA requests or not, I can't say.
Hope this helps.