Yes but what is the point of severity _and_ priority? Why not one field that's first estimated by QA and then updated by project manager when the clients needs are known?
If a base release that was customised for multiple customers has an expected number of P1, P2, P3, P4 issues for most customers but a high number of P1 and P2 issues for one particular customer, off of the same number of issues in the base release as measured by severity, then that will stand out in our measurements and we'll dive deeper into that customisation to see what's going on.
(Edited mid-stream... accidentally submitted too early.)
The point of this system is that high-severity bugs might have lower priority than low-severity bugs if you take business requirements into consideration. Yet, this does not mean that severity should be ignored.