I don't know why everyone else is just giving you other reasons.
Why use other accounts? To deceive! I'm not kidding. It's not the only reason, but it is a reason, and people do it. This is why limiting districts or matchmaking based on playtime and rank will never work. Because people will make new accounts just to smurf. We see in many other games, there's no reason APB will be any different. People absolutely will make new accounts just to game the system and get easier matches.