How do you know what is the montly search in bing or yahoo ? if you have 50k search in google that does not mean you will get same amount of search in the other engines just because not so many people use them, In fact, I personally don't know people who use bing or yahoo for search.... Also every KW beyond the first 5 spots on page 1 ...sometimes first spots on page 2 give you zero traffic. For example right now I'm ranking new KW with 1 million monthly searches but on #45 i see from google webmasters that I got total 12 hits (average 3 per day)