Another time fans called foul was this one, when someone they'd never met before was then retroactively said to be a good friend of Harry Kim's. If she was such a good friend, why had we never seen her before?
For one thing, Harry spent most of the series as the galaxy's biggest rube, someone who could only let loose around the irreverent Tom Paris. But that suggests that he otherwise didn't have a life before the first episode of the series, when we in fact know he had a fiancee on the other side of the galaxy, and therefore probably less socially awkward than he could sometimes suggest. If, then, he had other friends, chances are they were a little more normal than Tom Paris, generally speaking. Practically speaking, there's no reason to suspect that we should ever know everything a fictional character has going for them. After all how well do you know the neighbors you do know? Name the street they grew up on!
Anyway, just needling away at flawed logic now. People who want to complain about something will find more reasons than actually exist. The fact is, having a character in these particular circumstances exist as presented in a random episode makes a lot more sense than anything else the writers could have done. Short of killing off a main character and bringing them back (oh, wait, that's what "Fury" does a few episodes later) later on for a brief reprise, doing it with someone the fans had never seen before, but having a link to someone we had, makes all the sense in the world. We're learning everything about them as we need to know it. Emotional investment can only go so far in an episode that otherwise demands the viewer to ask more questions about the strange alien culture encountered than the character who's been forced into it. There have actually been other episodes like this, just not done this way.
"Ashes to Ashes" brings a lot of fascinating ideas to the surface. It may not be the greatest episode, and maybe someone else could have been the guest star's friend than Harry (imagine if it had been Janeway, or, again, Chakotay). But there ends up being a lot more said about Harry than anything else. And that's something Harry could always use more of.
franchise * series * essential * character
Memory Alpha summary.