Why software testing effort estimation is important after functional specifications finalization phase?

If we go by quality standards the sizing of software testing effort has to be done before the tester(s) start writing the test cases for the purpose. The estimate will clearly draw out of the functional specifications signed off between the customer and vendor. Without sizing the Testing manager can never create a testing plan based on which he will decide the number of days and persons required to write test cases, perform testing, draw out the testing results, submit the result report to development team and get the reported bugs resolved. The plan will comprise of time-line and no. of persons required for each of this phase in the sequence mentioned above.

To calculate a reasonable testing time-line estimate based on functional specifications there are certain guidelines that need to be followed: the person who is planning has to have ample business and testing knowledge. Unless (s)he has the right business knowledge (s)he will not be able to select the right persons for writing test cases, or able to guide them on the critical business rules and processes written in the software to hit upon. In that case the best of the test cases will lack the complete coverage and accuracy in testing. The software may lack bug-proofing at the end and customer will be the sole sufferer. Ultimately it is going to effect the software, and the organization that built it.