I have no real trouble distinguishing my [+] from my TBH from my HAF from my OPG...
They all stink like something died and crawled to rat heaven.
I don't think limiting unofficial groups would be an efficient use of Admin time. Often these unofficial groups start as immigrants from other servers/games who all want to band together under one banner, regardless of the politics of the server. I remember the LT group who came, for example, and who have now integrated themselves into the OPG. To attack groups like this is an attack against newcomers to the server, if they dare to put something by their name. In all honesty, it's easier for me to see an RA member do something wrong and mention it to Classic, rather than throw a hissy and post a sanction. Knowing the boss of the person involved in an incident is useful, if they didn't have a tag how would you know if they represented a larger body of people?
At the end of the day, it's easier to remember a tag relating to an Npc faction, than to remember what the enigmatic name of that pirate who attacked you some time last week was, and which faction he belonged to.
Who doesn't know that [RA] is Outcast, or that [GST] are a group of smugglers affiliated with the Outcasts? Well, in that case, can you guess who Pato.Dooley[Molly] is with?