This is, as John's answer suggests, a perennial (and divisive) question.
Brooks owners seem to fall into two camps: those who argue that as a tin of Proofide should last the lifetime of a saddle, there is little to be gained, and much to be lost, from using cheaper alternatives.
The other camp argue that Proofide is simply dubbin
, albeit Brooks' own recipe, a product which has been used for at least 500 years to treat leather (such as horse saddles) and that regardless of the "cost per application" there is no need to pander to the rampant profiteering and scaremongering of multi-national corporations (Brooks was bought by Selle Royal in 2002).
Can you guess which camp I belong to?
All the stories I have read of leather saddles being ruined are as a result of at least one of the following: becoming soaking wet and being incorrectly dried; over-tightening of the adjuster bolt; excessive application of leather treatment.
PS If I remember correctly, when doing my own research on the subject, Neatsfoot Oil was one of products about which I read bad reports.