it indeed sucks
By weighted average, I mean the countless times that users or blocks of users are down but it's not large enough (or in the public eye) to be counted as a technical downtime.
If inconvenience means that a business is crippled in supporting its customers or communicating with potential customers for an extended period of time than sure, it really doesn't matter.
It all really comes down to who you are. As an individual with the typical service level individuals demand, 99.9% (or even lower) is fine. As a business that relies on email as a critical mode of communication it is not an acceptable level.
As far as alternatives considering how inexpensive things are, with minimal effort and expense you can well exceed 99.9% managing an internal mail system. This would be appropriate obviously again for businesses of some size (say ~50 employees or more). Agreed that no system is perfect.
My point is that Google touts 99.9% as a great number and as someone who has had overall responsibility for managing internal operations of a small business (email included) I would have been fired quickly if that was my number.