You may remember that bad dps or person who stood in the fire, but usually they will be forgotten in a few days to weeks time. I cant remember people I ran with a few days ago. What I do remember is their guild. Guilds usually have a server reputation and a personal reputation (give by you of course).
With server reputation, you will pretty much bunch them all into one group. For example, if you see someone from the top raid guild on the server recruiting for a PuG run, you are probably more likely to try and get in than if you see someone advertising from a guild with all lower case letters.
It really doesnt matter that this person could be on a terribad alt. Name recognition goes a long way in WoW just as it does in real life. Is Pepsi really that much better than your local chain store's brand? Not really, but you pay for the label. You know exactly what to expect from Pepsi.
Despite sometimes not remembering soecific people, I will usually remember my experiences with members of that guild. If I have one really bad inciddent, you are marker.
Remember, when you are playing, you are not only representing yourself, but your guild. It is not fair to give them a bad name thru shady action.