There have been tons of threads/convos on this.
In the early days of Java 7 it was pretty unstable and there was pretty much no convincing reason to use Java 7 over Java 6.
Now that Java 7 has stabilized and Java 6 is EOL it's a reasonable decision to use Java 7 and we do it in production with no issues to speak of.
That being said there was one potential situation we've seen as a community where bootstrapping new node was using 3x more CPU and getting significantly less throughput. However, reproducing this consistently never happened AFAIK.
I think until more people use Java 7 in production and prove it doesn't cause any additional bugs/performance issues Datastax will update their docs. Until now I'd say it's a safe bet to use Java 7 with Vanilla C* 1.2.1. I hope this helps!