When Zoe first got glasses, we only bought her one pair (the sticker shock was enough that we just couldn’t imagine paying for two pair), but between prescription changes and replacing scratched lenses, she ended up having to go without glasses pretty often while the glasses were in the shop. So when she outgrew those glasses, we got her two pair of glasses, and I think she went nearly 2 years with almost no repairs, and no prescription change. I considered just going back to only one pair of glasses when she got new frames this time, but I’m glad we ended up getting 2 pair. In the 2 months since getting her new glasses, they’ve been sent back for repairs twice: once for a major scratch on a lens, and once for a defect on the frame (it looked like the frames were going to split in two lengthwise along the bottom of one of the lenses). I’m so happy she’s had a second pair so she hasn’t had to go without glasses. And very thankful for warranties so we haven’t had to pay for any of the repairs!
I will continue to recommend a back up pair, and warranties whenever possible when it comes to glasses for young kids.