Ideally, you want to be able to reproduce each bug so that, at the very least, you can test that it's been fixed.

But... That may not always be feasible or even physically possible. Especially with 'enterprise' type software where each installation is unique. There's also the cost/benefit evaluation. A couple of hours of looking over code and making a few educated guesses about a non-critical problem may cost far less than having a technical support team spend weeks trying to set up and duplicate a customer's environment exactly in hopes of being able to duplicate the problem. Back when I worked in 'Enterprise' world, we would often just fly coders out and have them fix bugs on site, because there was no way to to duplicate the customer's setup.

So, duplicate when you can, but if you can't, then harness your knowledge of the system, and try to identify the culprit in code.