Well people who impersonate other people usually do it to make the original person look bad. It happened a whole lot with MasterNoob. Obviously if the impersonation isn't hurting anyone, then no biggie. When people use the same name as mods, it gets confusing for other players, which is one of the reason it isn't allowed.
This. I can't tell you how many times I've seen Jake accused of hacking because of hackers who have used his name (including on TF2 Center). There have also been other impersonators of Jake who have done it simply for the attention (one was recently exposed on reddit offering to mentor somebody while claiming to be Jake, formerly of the Syndicate).
When I've seen fake bfl`Satan, he's joined up as Pyro, played poorly, and talked trash. Quite a few people were most likely left with a bad impression of the real bfl`Satan, who a fantastic player and person.
Maybe you could use the following criteria for allowing people to report impersonation:
1) Person is using the name AND tag of a different player
or
2) Person is using the name AND avatar of a different player
or
3) When asked if they are the player whose name they share, the person in question falsly claims that they are
I really dislike it when players use the tags of teams they aren't on, especially the higher division teams. I'm not sure if that would fall under impersonation or not, however, a player doing this could damage a team's reputation.
You'd probably would want to make it clear that people requesting action against impersonators should check the links for the user's steam profile and UGC and/or etf2l history before reporting; it's those places where you can see most of the story. You'd also have to make sure the rule covered times when people deliberately added a small difference to their impersonation, such as using "StabbyStabbey" or something.
Again, wouldn't want to see harsh penalties, except for repeat offenders.