What's the idea? To have two rot values for an item, one of which can be repaired and one of which can't.
What does that mean? That items will have a fixed "life". If you don't repair an item it'll still rot before the end of it's fixed life, but you can't keep repairing that bone knife indefinitely.
What will this add?It adds value to more expensive items (the knife over the bone knife, for instance). It also means items cannot last for ever and new ones must be made, which adds an element of consumption to the game which is sorely lacking at the moment.
Will it just be tools? In the short-term, yes. However, this idea could be used on machines and buildings.
A cosmetic point - how will it look? The system would introduce a new system of naming on two scales. (Well maintained-maintained-fairly maintained-poorly maintained-crumbling & Brand new-new-standard-old-antique for example. The naming needs work.

How will it be balanced? Obviously this idea will require a large amount of RD time to create a balance. That is a large project, this wouldn't be brought in immediately, but the idea would be to balance the complexity of a tool against it's end-life.
I'm happy to answer questions, it's always good to develop an idea. So, let the criticism commence!