I think that reusing code is probably one of the best - and worst - things since sliced bread.
It gets the job done quicker. That is certainly a huge plus - after all, many clients or even employers require quantity above quality (sad, but true). By being able to reuse your code, you can cut your work load in half. On top of which, it means that you have your own blueprint for sites - also meaning that you can concentrate more on the actual design.
However, it can make it so that you limit yourself. If your code isn’t very well documented, you might not know what’s going on where, and end up spending more time troubleshooting than actually designing - definitely something that is quite bad. In addition, you can start churning out designs that all look the same - meaning that your skills as a designer can - and if you keep at this long enough, probably will - degrade over time. I’ve had this problem before.