This is only reasonable to a point. You're essentially doubling your costs because things don't exactly work the 'same' between zones. It's not like you're putting a bunch of servers in the same platform exactly and spreading them out around the planet.... so you can't have four web-servers instead of four and send traffic two all four (with two sets at another AZ) without considering and also having a separate DB setup in that AZ, etc.... so costs quickly mount (not to mention it's a much more complex platform suddenly).