So "eventually consistent" networks are A+P, but compromise on C? I guess that for many applications it's good enough.
The main tradeoff is that after writing the values 1, 2, 3 in order, reads could see anything from no value or any one of those three values until the nodes converge.
In a Consistent system, if a read happens after writing and you see a 3, you will never see a 2, 1 or no value on subsequent reads. In the case of a network partition, the system will prefer to not be available than to return reads older than reads that have already been returned.
However, I was under the impression that this is not the "default" and I am likely to get performance poorer than distributed systems designed to be Consistent (like HBase).
It's true that QUORUM reads will have half (in a 3-replica system) the throughput as ONE, but with Cassandra reads up to 8x as fast as HBase [1], it still wins handily.
Finally, the reason this is worth making configurable in the first place is that almost all applications do just fine with most operations at ONE.
[1] http://vldb.org/pvldb/vol5/p1724_tilmannrabl_vldb2012.pdf