There's nothing wrong with shooting a shotgun like a rifle with slugs.  The slug is a real thumper on the receiving end.

 

The smooth bore shotgun is a dual purpose firearm.  With shot you can bring down all manner of game from birds to smaller critters to even deer with buckshot (hence the term).  The problem with the shot is that its range is less because the multiple projectiles spread out with range and the striking energy decreases with number and size of shot as the range increases.

 

That's why, with shot shells, the shotgun is considered a short range firearm.  Remember that the shotgun was never intended to replace the rifle.  If you want rifle-type accuracy, then use a rifle.  The shotgun can be used with shot or slugs, but always remember you're shooting a shotgun and don't expect it to do something it can't because of the kind of gun it is.