[ineedasername]

That is a premium price if you're using it 40hr/week. Although, asumming they're available for custom AMI, maybe the value-add use case for this is to be able to have multiple custom AMIs for different dev environments and scenarios to rapidly spin up what you need.

Specifically for testing with multiple versions of MacOS: you could develop the code and then test on a custom AMI to make sure it ran in the version of MacOS, no need to have multiple systems setup. (Although I don't know much about Mac dev-- does Apple provide tools to do such testing already without running multiple OS versions?)

Otherwise, I guess it's all just about convenience.