Pros are that it filters out much smaller particles. (Down to 2 micron) which can reduce engine wear and extend life. I like the idea that they can filter out soot and carbon particulate, which are becoming a problem with direct injected engines, and tend to wear timing chains in particular. And you get some extra oil, which is also a con.
That's really it. Sure you could argue it extends the life of the main filter...But no one's really gonna run longer on the main filter, and many people couldn't anyway as they already run out to the max life of the filter.
The drawbacks are cost, of the initial outfitting. The extra cost of oil, the extra cost of the bpyass filter, the fact that it's large and not particularly easy to find a protected place for it, and therefore the potential complexity of an install, as well as the additional failure points introduced into the piling system.