Sure, a single missed day isn't bad; but have you ever skipped a month?
I'd been using Anki for Mandarin flash cards for a couple of years, and decided I wanted to memorize the "outs" (probabilities) of various hands for Texas Hold 'Em Poker. So I made a separate Anki deck and used it for a few months. Then I got busy, and stopped studying the poker deck (while maintaining the Mandarin deck). When I tried to pick it up again, it was just impossible -- I'd forgotten so much, and the card list was so long, that if I said "I forgot this", it would be scheduled for the next day, but because it was behind 100 other cards, I wouldn't actually be shown it for a week or two. The system was just completely broken.
Contrast that to the system of my own that I developed, based on the "study value" (effect of studying now on the difficulty) rather than fixed timeouts. After working on and using my own system for about 4 years, I got a bit tired of it (and I was also in the middle of redesigning the database from the ground up), and so decided to give Duolingo a go for a bit, just to see what it was like. Six months later, I came back to my own system, and it was great -- just slowly eased me back into the vocab I had before. The same is true after missing a day, or a week: it's always welcoming to get back into, rather than terrifying to get behind.