It really depends on what you want. If you're totally happy with 5.3.3.1 and don't feel compelled to upgrade, I wouldn't necessarily try to convince you otherwise.

However, I will share why I upgraded my sites, and then let you make a decision from there...

1. Bug fixes. Even if I'm not aware of them, I may come across them one day and upgrading might prevent them from causing problems with my site.

2. Blocks/Packages. There may be a block that comes out that requires the most recent version, and it would be nice to not have to worry about upgrading then.

3. New features. There might be features that can really help you manage your site that once you start using, you'll wonder how you got by without.

4. Getting help. Most of your dedicated C5 users, including the core team, are going to be using the newest version and will have that on the brain much more than older versions, so help with older versions will be harder and harder to come by. This probably isn't so true at the current moment, but could be once we're on Concrete 5.5 or 5.6.

Those are just a few reasons I upgrade. However, I'll confess I've still got a few sites running 5.3.3.1. I also stumbled across one the other day that was running 5.1!

Check out the release notes and see if there's anything that makes you really want to upgrade.
http://www.concrete5.org/documentation/background/version_history/v...

If not, I wouldn't feel too bad for not upgrading ;)