Factory Testing is one the terms that leads to a lot of acrimony in the industry. There are people who believe it and will evangelize for it and others who claim it stifles every innovative thought ever made.
Some of the people who believe in it fervently are those who have exacting expectations from their clients. They need to be able to tell them with certainty what tests were executed, when they were executed and what the results from those tests were. At the end of test, there is no question as to what has been done and the decision about whether to proceed is easy.
Those who believe that this process stifles creativity think that simply following fixed scripts without thought reduces the role of a tester to that of a checker and not a tester and the work might as well be done by someone without a testing background.
There is room for both views but they need to be applied at different points of the Software Development Life Cycle.
Creativity needs to applied (in both views) at the time of Test Case or Test Objective Creation. We certainly need creativity and original thought in determining what to test and how to test it. Once that has been done and the final client has agreed that the coverage is sufficient and then the paths diverge a little, but not as much as some people think.
In the Factory Testing method, the testers (or checkers) must finish all the tests that are approved in the way they have been written. This does not preclude them discovering other ways of testing and new tests that can and should be completed. it just means that they may not necessarily complete them immediately for the current cycle or release. They certainly should not be lost and can be used to improve the coverage and process next cycle or release or even in the current period if they are important enough.
In the other methodologies, we can still use the approved tests as a starting point and then start to add and expand. However, we do need to know the best way of augmenting tests while ensuring coverage. Quality Assurances focus on statistical analysis will allow the calculation of those figures and make that decision.