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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)

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.

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.

Multi-AZ is a requirement on production level loads if you cannot sustain prolonged downtime.

Datacenters do end up completely dying now and then, you really want to have a good strategy in that case. Or not, if that's not required.