There are many, many, many arguments around upgrading Enterprise Vault or not. In fact the same is true of any application or Operating System. I am a fan of upgrading quite soon after a release of a new version or service pack. There are numerous benefits.
Of course I would always suggest that you do adequate testing in your own lab environment first - and yes, it's hard to get a lab environment which matches your production in environment in terms of scale, and throughput. I wrote an article about why you should have a lab, take a look here. One of the key reasons for upgrading though is all those 'little' fixes that help clean up your Enterprise Vault event logs. Issues like the one described in this technote: