There are different schools of thought on this. The author advocates implementing some flexibility in a formation contract, including setting aside some equity for whoever emerges as the dominant contributor or some future reallocation. On the other hand, there's the view that any up-front contract should be iron clad and detailed so that the parties expectations are aligned, and any ambiguity just leaves open a possible dispute. I'd imagine a lawyer would subscribe to the latter view.
Thought this crowd would be interested since this comes up in the context of an equity split among founders almost always. Interested in hearing opinions.