Everyone in favoring of getting it in writing has made very good points. I'll just add this one:
Forcing it to be put in writing makes both/all parties think through all of the variables, what-if's, and what-could-go-wrongs. In other words, it "surfaces" issues that would otherwise stay buried, waiting to strike later. By committing the agreement to writing, yes, you may have to have a difficult discussion or two with a neighbor, family member, or partner, or whoever is across the table from you.But better to have difficult or complex issues sorted out upfront, than to be torpedoed later.
A final reason is that (depending on the state, etc), some types of contracts must be in writing if you need to actually sue to enforce it if it all blows up.
Put it in writing. It needn't be fancy, just thought-out and clear.