The only way I could think of this being abused would be someone subscribing to tons of people, hoping they subscribe back? But that would be hard to do. Also, a minimum requirement of 5 quizzes, 25 levels and 50 subs or something to get on leaderboard would stop that.
Other question, does having more subs lead to new quizzes ACC being played more?
Yes, having many subscribers gets you takes because they get a notification every time you release a quiz.