Because we operate in likelihood ratios, the unexposed risk is important so the risk can be computed (at least, in theory; in practice, people go more by indication than by formal risk prob/benefit prob).
If I want to know whether to do a CT scan, the only relevant fact is the increase in probability (in an absolute sense) of cancer that it causes.
It seems like either you, or the entire field of medicine, is obfuscating this basic fact using some field-specific concepts.
EDIT: It seems like you are referring to relative risk (http://en.wikipedia.org/wiki/Relative_risk). But as I said, there is no reason that relative risk should be considered in this case.
