If you have sufficient surface agitation from your filter, you don't need one. But, nonetheless:
Pros: some believe they increase oxygen levels (I don't), a fine stream of bubbles hidden in between the plants can look quite nice (not attached to a spongebob or volcano...)
Cons: motors can be very noisy and vibrate against things, use electricity you don't need to be using, the spray from the bubbles popping can get all over the lid and lights.
Not a fan,as you can probably tell, but if you like the aesthetic effect of them, by all means get one. They do no harm. But adjusting your filter output is free and quieter.