The main pro of this for open water divers is that this is a more comfortable configuration for a lot of people. Instead of having an single 80 cu in tank on your back, you can divide the weight and have two smaller tanks on your sides. You can also take the tanks on and off in the water (and you can carry a lot more tanks on a sidemount configuration if you need to). It also allows for a wider variety of configurations so you can really customize it for your comfort. What I like most is that I feel a lot more balanced when I dive sidemount and it’s easier to keep a good horizontal position in the water.
Here’s why I haven’t completely converted to sidemount:
If you want to don your stuff in the water, it’s nice if you can get your car close to the dive site or you’ll have to do a lot of schlepping. (If a hike is involved, I prefer just plain old back mount.) Diving from a boat also requires extra work - a lot of people like to tie their tanks off to a line and don and doff them in the water (a backwards roll wouldn’t work). Finally, you’ll have to invest in two of everything - first and second stages, gauges, etc.
Also, as a new diver, you have to remember to change to breathing off the other tank every 500 psi or so to keep from becoming lopsided - especially if you are using aluminum tanks which become lighter as they empty. To do this, you’ll have your two regulator hoses draped over your neck. I felt a little confined at first and it took some time before I felt comfortable.