Earlier quoted context omitted.
And then you get slow writes and/or reads as multiple nodes need to be contacted, as well as temporary outages when a quorum cannot be formed.
That's a pretty silly objection. With the common three replicas, I have to lose two Cassandra nodes to be unable to achieve quorum. I have to lose just one regionserver to be unable to read from HBase.
When we have R+W>N, and assuming N is 3, there is either a hit on read and write (W=R=2) or heavy hit on write (W=3, R=1) or heavy hit on read with a chance of temporary outages (W=1, R=3).
In HBase you write to exactly one server and read from exactly one server, which also provides for cheap atomic operations.