What are the advantages to these?

 

I was wondering the same thing

 

It depends on the 4 wheeler you are talking about for instance a 650 Prairie has a mono shock solid rear axle with a single sway arm in other words if one tire is in the air and it is sitting level ( not leaning to one side) than one tire has to be in the are it does not alow the rear axle to twist.

Now on the other hand Suzuki and Arctic Cat have a dual shock rear end (in my opinion is the best set-up) it has a solid rear axle but acts like an IRS they use Sway arms much like automobiles do it allows for the rear axle to twist without loosing traction on both rear wheels. It uses a 4 point connecting system to allow it to twist, instead of a hinge like on the singles shock.

Is this what you are asking about???
Hope this helped if not go look at the larger Suzuki's rear and compaire it to a sport quad rear. You should be able to see the differance.