This is acceptable for a wide range of apps. EtherPad, for example (which used MySQL) only flushed every second or two. If the database crashed, then yes, you'd lose the last couple keystrokes, but that was so extremely rare it didn't even register as a factor in the overall UX.
Edit: Also, you probably have bigger problems if your database goes down or your cluster falls off the map somehow.
Absolutely, but people should still be aware of the compromises and not confuse categories.
Very few apps can make do with only a key-value store and if you have to throw a relational db in the mix, why introduce more moving parts unless absolutely necessary? ACID, referential integrity, SQL. Sometimes it feels people are willing to ditch all this for very little gain.
Let's face it, how many apps out there can't really run a beefy relational DB with potentially an absurd amount of RAM?
Edit: Also, you probably have bigger problems if your database goes down or your cluster falls off the map somehow.