The first time a power dropout or interruption causes your digital board to reboot, you'll really wish you had a UPS on the board. That 30 to 60 seconds of booting back up will seem like a REALLY long time.
+1. Had a child pull the FOH cable at a festival last year. It's amazing that I got away for a decade without that ever happening. Went out and bought a UPS. You don't need a very large one to feed a digital console. The downside for me, more than the cost, is the extra weight for a rack case and UPS (35-40lbs for a small UPS in a wood case).
On the flip side, now that I have a digital snake my DSP is on stage, so I really need 2 UPS systems, although the DSP reboot is faster than the console.