Failure to see applications is a failure of imagination. Also, Cassandra is highly optimized for extremely fast write rates at large scale, not read-only. Cassandra also has no SPOF, includes variable consistency guarantees, and many other great features. Specifically focusing on the fact that it can have 2 billion columns in a row glosses over a number of other things, but that hardly denormalized or non-transactional databases have few applications.

I use a branch current monitor from PowerLogic (BCM42) coupled with a Barionet-50 to perform monitoring of every circuit in the house simultaneously. Some details can be found here:

http://blog.insidesystems.net/modbusrtu-via-tcp-serial-gateway-with-ruby

The kinds of analysis you can perform with this are tons of fun. For instance, how much does that desuperheater in my geothermal unit really help the water heater? Has a lightbulb in one of my floodlights gone out? Did somebody leave the refrigerator door open? Has the water softener stuck again causing the well pump to work overtime? This approach lets you monitor WAY more than just consumption.