The variables are the same as those for on base percentage (OBP) for batters, which is something that is familiar. Why not use OBP? Why dream up something else and put it into a pitcher's context that requires translation to have meaning?
If I say that a batter has an OBP of .375 you know that is good but not great. If I say that a pitcher allows an OBP of .375 you can infer the same thing in reverse: bad but not terrible.
But what if say that a pitcher has a WHIP of 1.4506? What the heck is that?
It's career position number 978 and it belongs to active pitcher LaTroy Hawkins. It is terrible but if someone woke you up in the middle of the night and asked you about it, aside from being upset at being awakened, would you relate to that number? Probably not.
Plus, WHIP does not add anything to OBP. We have long had the data to compute OBP for pitchers. Slugging average is different as we have not had doubles and triples allowed by pitchers.
Should WHIP be adjusted for park and year? Yes, but so should OBP.
Except for ERA (earned run average) we should use the same equations for pitchers that we use for batters.