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.