Let me play devils' advocate -
What does your gentleman's agreement say about antiquities?
Let's say you have a main contractor on site, they start excavating, and holy moley they dig up a norman chest full of gold amulets.
Who do they belong to?
Contract Law would state that arisings and materials from carrying out the works belong to the Contractor! They get one sniff of that law and they will fight you all the way for a few million don't you think?
Just saying.
I agree with the fact that on the vast majority of well-run sites it's not required, but wow does it help when you need it. Think of playing football without a rule book - silly isn't it? How many times when you were young did you all know the rules of football but still get into an argument about who did what? I wouldn't use a contract to enforce silly rules, but certainly, if my client was at considerable risk then I would.
Great debate guys! I guess the moral of the story is that there is a time and a place for it, and it would definitely depend on the way in which it was introduced, and who was administering it. You need to be fair to both parties.