It appears that attitudes on mining the lesser used, but more specific “long-tail” phrases is changing. Quality Score is making it too expensive to go after these phrases. What this means is to improve the performance and lower the cost of your campaigns you need to clear out the deadwood.
If you must stubbornly clinging to your long-tail phrases, move them their own group so they aren’t needlessly impacting the Quality Score of your other campaigns.
More on this topic here in Yahoo’s Search Marketing Blog titled:
Chasing our Long Tails.