Old games used a trick where it was a "shadow realm" or something, which could be annoying.
More recently, we had Dragon Age 2, which I thought was a really good game--except that it had no chance because people hated the constant reusing of the environments so much. I found that to be annoying and tiresome but didn't hold it against the good aspects.
Overall, I don't consider it to be as big a deal as some, though it depends. If the developer leaves the exact same enemies using the exact same spawn data, then we have a problem.