I would say an emergency spillway is always a good idea. Here in Bryan, we got 18 inches of rain in 12 hours about 3 years ago. Granted that is extremely rare, but if you get hit with the residual from a hurricane just right, you will wish you had. Spending a few hundred to a thousand on a dozer to make the spillway is money well spent in my opinion. Even if it costs more, you will be able to sleep when it is storming outside at night.

Water dries, rocks crumble, and trees die. The only thing that is eternal is the reputation we leave behind.
- Ancient Viking Proverb