I've shot shot through some of my slug guns in the past, never liked the patterns much for anything but self defense....It's not dangerous in any way, it actually should have LESS barrel pressure than as fired through a standard shotgun bbl, it just gives you really crappy patterns.

If the rifling does indeed impart rotation on the wad and shot cup, instead of just carving away parts of it as it leaves the bbl, the shot, as mentioned above, will get spun out pretty substantially. I'm not going to say that it's necessarily all to do with the rifling however...I'm sure it doesn't help anything imparting the spin on the shot, but...

If memory recalls, most slug bbls are cylinder bore, i.e. NO CHOKE whatsoever, and they typically have a shorter bbl anyway, usually between 20-24"....so combine the centrifugal force driving the spread radially, and the fact that the broad choke and short bbl would have a rapidly spreading pattern even in a smoothbore, you just DON'T have a good gun.

Aftermarket replacement barrels are available for most slug gun models anyway.