A lot of really good info in here, and a nice thread topic :dblthumb2:
Here is my view.
Firstly, the test spot should be at the forefront of everyone's mind when selecting a product/pad combo. It is very easy to walk into a detail with preconceived ideas about your plan of attack only to have your test spot turn that on its head.
So, if you need to let the paint guide you on what combo to use, and you logically intend to start with the least aggressive combo and work your way up until you remove just enough paint, but not more, then you need a 'range' of products. Pads which increase in aggressiveness, and polishes that do too, so logically there is always going to be a need for these various 'grades' of polish.
Perhaps PF2500 does sit too close to SF4000 in terms of cutting ability - let's just assume for illustration purposes it sat more in the middle of SF4000 and FG400, and you were working on a car with plenty clear and deep damage. You complete a test spot with FG400 on your most aggressive pad and it hasn't removed all the swirls.... most, but not all.
You have to do a second step but SF4000 doesn't have enough pull to remove the remaining swirls, regardless of which pad you use. If you did not have an 'in-between' polish, you would either have to do a second polish with FG400, which could potentially remove more paint than was necessary and not finish down as well as you finishing polish, or go FG400/SF4000/SF4000?
Will this happen often IME, no. In most cases I have been able to do a 2 step correction in the traditional FG400/SF4000 route, but there have been occasions where I have gone FG400/PF2500 (a little extra swirl removal was more important than absolute gloss).
I also really like Mike's suggestion at going from a compound to an AIO. If the AIO has enough pull then it can remove any compounding haze, and lays a month's worth of protection down in one step, and the owner intends waxing his car every month anyway, then the initial durability isn't the be all and end all of the detail.
So going back to my original point, to me, EVERY detail is different and so to create an individual route to get to the unique objective of 'that' detail, then having a variety of products at your disposal makes that easier to achieve.
Let the force (of the Flex:buffing

be with you and the paint guide you :laughing: