What is the importance of using a pKa value instead of a pKb value when describing drug chemistry?

Why do we only consider the $\mathrm{p}K\_\text{a}$ value of a drug regardless of whether it is an acid or base? Why do we not use the $\mathrm{p}K\_\text{b}$ value?




For easier comparability: 


Comparing two pKa values is much easier than comparing a pKa with a pKb value.


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The reason we only usually consider the $\mathrm p K\_\mathrm a$ is that in a constant medium (in the case of drugs, water) there is a simple relationship between $\mathrm p K\_\mathrm b$ and $\mathrm p K\_\mathrm a$ so we don't need two scales for acidity and basicity and can define a continuous scale from strong acid to strong base using just one number. 


$$\mathrm p K\_\mathrm b = \mathrm p K\_\mathrm w - \mathrm p K\_\mathrm a$$ where $\mathrm p K\_\mathrm w = -\log\_{10}([\ce{H+}][\ce{OH-}])$, roughly equal to 14 under normal conditions. 


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