If you are going to add remotes to your 3215, I would suggest you purchase your components from online or local sources versus getting the remote kit from your dealer. Way cheaper, and you can add a 2-spool valve instead of one. When adding the second control valve, you feed it thru the Power Beyond port of the first control valve (you cannot place a 'T' fitting in the incoming hydraulic line from the hydraulic block). Most of the manufacturers, such as Prince, have diagrams on their websites showing the proper plumbing configurations.
Advantages? I can hook up any piece of equipment to my 3-point hitch without wasting time or hasseling to line the top link up. Very quick. Biggest advantage is when using the boxblade or scraper: I can adjust the cutting angle with the toplink cylinder on the go. With a boxblade, this is a huge advantage, as this effects how deep you can remove the surface dirt, gravel etc, or just smooth it for a final grade by adjusting it in the opposite direction. In addition, by being able to adjust the horizontal tilt, you can maintain a crown in gravel driveways, which is critical to keeping erosion from washing away across a flat driveway. There are many other uses for this add-on; you will find many other small advantages as you use your equipment. Is it worth the money? I'm convinced it is.
I'll post more pictures in a few days highlighting the plumbing to the second control valve and a few other details.