I did find an online reference on Facebook from a bike shop that includes the following, in its FAQ on Brooks and Proofide. I have no idea if they're up in the night on this or not, but here's what they said:
"HOWEVER, in both of these two cases, it is very important to be mindful of the irreversible damage that can result from excessive application of Proofide. There is a risk that the leather will over soften and then no longer be able to support the rider’s weight."

Whether this is a statement supportable by evidence of it actually irreversibly damaging saddles by over-softening or not, I have no idea. This could just be some of that infamous Internet conventional wisdom for all I know.