DISCLAIMER: This post violates every YNAB rule imaginable, takes a lot of discipline, and will likely get me flamed. But here goes anyway.

If you are "daring" and feel you have the discipline to do it, the best "buffer" is a credit card you pay off every month. If you can put most of your expenses on the credit card, you can have zero buffer, yet still not have to make multiple trips to the bank per month. All of your expenses can build up and then be paid at once when the credit card bill is due.
The danger with this method of course is that you can easily overspend and carry a revolving balance. However, if you are using YNAB to make sure you stick to your budget and don't spend more than your income in a given month, this will not happen. Of course, overspending will inevitably happen, so SOME buffer is nice (otherwise you have a small revolving balance until you can compensate for the overspend in later months). But a full month's buffer is not required.
Let me reiterate that this is not a desirable long-term goal, but while you have debts to pay off, it can help you pay them off faster and incur less finance charges on them by using most of the month's salary that would be sitting in your buffer instead to pay off the debt. Thus, I am pretty much agreeing with your husband that Rule #1 is not more important than debt reduction IF (and that's a big IF) you can be disciplined and live with a small buffer and deal with the fact that an overspend may mean a temporary revolving balance on a credit card (which should be fine as ong as the rate isn't much higher than the other debts you are paying off).
What I've found with YNAB is that there really is only one rule that matters in the long run, and that's to spend less than you earn. YNAB is very good and giving you the tools to achieve this goal. If you do that, you will have extra money to put somewhere. Building a buffer IS a good thing, and may USUALLY be the best thing for MOST PEOPLE, but I don't think it's ALWAYS the best thing for EVERYBODY. Some people may be better using that extra money for paying off debts or putting into into a high-yield savings account. Other people may even want a buffer larger than one month. I'm sure Jesse and Co. created the 4 rules to be a good guide for most people, but surely it's not the same for everybody based on different people's situations and how they deal with them. Regardless of which direction you go, it is a good thing to have a buffer in the long run, regardless of whether you decide to build it up now or after your debts are paid off.
As long as you stick to your budget, there's not really a wrong way to go. Good luck!