It depends on the stage of the business. For a small business that might be running off of one or two VPS's, you're probably much better off paying a few hundred dollars a month for a machine that is up 99.99% of the time instead of 99.9% of the time than to spend significant engineering time trying to setup database replication and failover (and either relaxing consistency or adding significant latency to your site) in the hopes of getting an 99.999%.
This is especially true given that more often than not, within the first 5 times you use it, you'll experience some bug in your HA setup that leads to some complicated, slow-to-diagnose-and-fix failure.
This is especially true given that more often than not, within the first 5 times you use it, you'll experience some bug in your HA setup that leads to some complicated, slow-to-diagnose-and-fix failure.