Suppose it's time to setup multi-az and pay to insure against AWS' own failures. I don't know why I previously thought their EC2 uptime claims were sufficient. Lesson learned.
Are you sure you understand their uptime claims? They offer a 99.99% SLA for regional availability, but only 99.5% for individual instances (and even then, they only owe you a 10% service credit for affected instances)
99.5% availability allows up to about 3 and a half hours of downtime a month. 99.99% means around 4 minutes a month. So if you can't handle hours of downtime, you should definitely be multi-AZ.
What are you hosting? Multi-AZ seems like a bare minimum for basic reliability. That said it's not a panacea. There's all sorts of cascading/downstream "weirdness" that can result on AWS's own services through the loss of an AZ.
https://aws.amazon.com/compute/sla/
99.5% availability allows up to about 3 and a half hours of downtime a month. 99.99% means around 4 minutes a month. So if you can't handle hours of downtime, you should definitely be multi-AZ.