HH - absolutely agree that the RN exists to do more than fight wars and the 99% number. I remember those elements well from the original FSC requirement and ST(S) exercise in 1998.
However, ultimately the RN is required to fight wars and in a time of limited resource, diluting warfighting capability on the basis of a somewhat specious argument about "fighting pirates with T45s" does not make much sense.
Were the argument to add hulls on top of the 19 DD/FF then that might be a different story - depending on whether one believes that 19 is sufficient to meet national tasking. However, I don't think anyone believes that that is the case being made.
One might apply a similar argument to the cost-effectiveness of using two-seat FGA designed and outfitted to penetrate IADS to deliver shows of force and PGM on Herrick. I don't see a huge clamour to acquire something like the A10, or the jet that shall not be named, or modern versons of the A37, Strikemaster or even A1 to service the Herrick requirement, do you? Certainly not at the expense of existing FJ force structure.