You seem to be coming from a corporate viewpoint -- if you're just renting something from a one-off landlord, the rental agreement doesn't at all have to cover every possible scenario, and doesn't even have to be in writing at all.
The problem with oral agreements is that even with the best of intentions, it's difficult to remember what exactly did you agree upon many months ago, so, although an oral agreement for a residential lease is still completely legal, and would indeed be accepted by a judge in California, a written one would be much easier to enforce and present to the court (even if the whole agreement is done through email -- you'd simply show the email (or emails) to the landlord and the judge etc).
Things that are not specifically covered by the rental agreement, in the United States, would fall under state law. For example, in California, unless otherwise agreed upon, you have to pay the rent at the end of the month -- which basically noone ever does, so, hence most agreements do explicitly specify when rent is due, since this specific part is always important to both the landlord and the tenant.
It's always best to try to get everything done through mutual agreement. On the other hand, if you think you might have a nasty landlord that's trying to abuse their power and discretion, best course of action would be to go to a law library and look at the relevant state laws (in annotated form), with summaries of sample cases of various past disputes between tenants and landlords (called "case law" in legal jargon).