It's not necessarily different than using a normal RDBMS, right - you could do a check in SQL outside a transaction and end up writing multiple times. But with an RDBMS, you can easily solve the situation by turning on a transaction and leaving no question about things.
This is why things like VoltDB ("NewSQL") are pushing to keep SQL and ACID, and figure out a way to scale, instead of throwing it all aside and making the developer deal with consistency issues.
It's not that you can't end up with the same functionality using eventual consistency, just that it's harder. Just look at Amazon's "apology based computing" (I think that was the name) and how they structure their systems to be resilient by being able to resolve multiple conflicting commands in a proper way (deciding, without communication, which command wins, figuring out rollbacks, etc.) It's fantastic, and perhaps it's the only feasible way to operate at their scale. But it's also a hell of a lot more complicated than "UseTransaction = true".
(So my predictions/guesses: If developers that'd otherwise use a traditional ACID RDBMS switch to non-ACID (BASE?) systems, they'll end up introducing bugs due to the shifted responsibility of handling data consistency. And seeing how big servers are, and even how far sharding can take you with normal RDBMS, the scale at which people "need" to drop ACID is probably far higher than the point at which people are dropping it.)
