Well Yahoo has got more traffic volume, but the reason why some people my want msn is that it gives you more options to filter your results
For most search queries in the US, Google is No.1, Yahoo is No.2, Bing is No.3 But different keywords and different countries may have different results. For example, in Taiwan & Hong Kong, Yahoo is too much better than the other two.
I think microsoft has captured a bigger market but yahoo is also doing good and giving tough time to MSN.
Yahoo has more traffic but Bing could be getting more. As Microsoft is running yahoo now or will be soon. Advertise on both different search engines bring in a different crowd. Msn may be good for one product and bad for another as with yahoo.
I get 5 times more traffic from Yahoo than I do for Bing. And this is with first position placements on Bing.
You must use both. Depending on the niche, both Yahoo and MSN can be exact equals as far as your conversions go. I cant make a statement that lets say Yahoo is better than MSN or vice versa as both are of great benefit if used properly!