Just to clarify, the reason it is important to do this upgrade is because there are security holes in the original evasi0n jailbreak which make it possible for malicious code to run as root. Essentially, it completely removed the sandbox. There is debate in the community about whether this is a flaw in evasi0n (comex' position) or typical of every jailbreak (evad3rs' position). I don't know who is right, but it seems wise to update to close these holes now that they are known.