I am sure you are right! However, I think another important factor here is how accurate people need to be, esp with the use of VC materials. If at 20 degs C my correct dev time is 10 mins and I use 11 at 19 degs (whereas the correct time should in fact be 10.75 or 11.5, it is neither here nor there for VC use.
Can you explain whether making a 10% change for each degree could be more wrong than making no correction. I know this sounds daft, but say at 20 degs C the time should be 10 mins and at 22 degs C I use 8 (whereas the correct factor dictates it should be 9.1, I have made a bigger error by applying correction than none at all.
I guess one needs to use teh provided slopes unless one is using a bespoke developer or one where such things are not supplied, such as pyrocat.