The biggest pro is that, as long as you are going to have some sort of covering garment on anyway, you don't need to adjust your wardrobe to accommodate the gun. You can just throw on the whole rig, spare ammo and all, and not have to worry about whether or not your gun belt is too wide for your belt loops, or if your IWB will work with the pants that you have on. The Jack Ass will accommodate any barrel length among guns with the same frame, so I can use the same rig for a subcompact or full-sized piece.
The draw speed disadvantage is overstated, as compared to a strongside belt holster the draw is slower only if you are standing and facing the threat. So, yeah, in Dodge City the shoulder holster is going to be at a disadvantage, but if you are seated, kneeling, belted into a vehicle, etc., like a lot of us are for long stretches of most days, the shoulder holster is easier to get to. Also, consider that the shoulder holster draw can be prepped, by grasping the gun butt while crossing the arms, without "drawing" any attention (pun), and you really have some advantages.
I've seen some people rigging their shoulder holsters so they work more like a cross-draw belt holster, but I like both sides of the rig high and tight, so no tie-downs are necessary.
The biggest downside that I see to them is finding a place for live-fire practice.