Because you lose accuracy.
There are spots in my flats that can only be mitigated with blur radius values of around 85. I had to run a few tests to figure out just how bad values like that affect the end result. As it turns out - not that bad but it’s not like having the blur radius at 3 or even 30. There IS noticeable difference and when it comes to stitching panoramas ANY noticeable vignetting is bad.
RT is great at removing vignetting, and I do mean that. However, it needs some basic spot removal tool badly. Both, to be used in the photos and on the flat fields. Being able to actually clean up the dust in the flats would be so much better than having to bump up the blur radius, which is nice that it’s even there, with the values allowed to go up so high.
So whoever is developing RT: Spot removal tool ASAP, please. It should be there, even if it’s the only “local adjustment” tool. Frankly, in my workflow that is the only tool that I miss in RT.