I'm definitely a non-traditionalist when it comes to fantasy categories but I still prefer WHIP over K/BB for a few reasons.
1. As noted above K pitchers are already rewarded for their high K's with Ks as a category.
2. As noted in some existing research
, K/B ratio is not a linear ratio with respect to its impact on ERA or pitcher effectiveness. What I mean by that is that two pitchers, one with a 10K/5BB per 9 innings and another with 2K/1BB per 9 innings will both have a 2:1 K/BB ratio. However, all other factors being equal, the 10K/5BB pitcher will be more effective and have a lower ERA. This is because each K has a greater positive impact on run prevention than each BB has a negative impact.
So using K/BB ratio would reward a pitcher with 2K/1BB more than a pitcher with 9K/5BB even the the latter is a better pitcher.
3. Finally WHIP is a fairly flawed stat since it is highly dependent on luck and defense. However more sabermetrically minded managers can use this to their advantage by selecting players who are currently experiencing bad BABIP luck, players who move to better defensive teams (or whose team defense was improved in the offseason such as the 2008 Rays/2009 Mariners), etc. This knowledge can give you a leg up on more ignorant league mates.