There's a few fixes. And there is the security patch, so yes it definately is worth upgrading. It takes five minutes, since your old skin will work perfectly without any modifcation (in 99.9%) of cases.

How dated does a version have to be in order to warrant a complete reinstall?
Im still running the old .3 version (doh!) and i know an update is way overdue. But can i still keep my old skin?

You mean 0.9.0.3?

Short answer - probably. You may need to make a few minor fixes though. A good diff / merge utility will help loads - http://winmerge.sourceforge.net/