I know this isn't a question for spoons, but this will make sense in a minute:
With spoons, there are a few companies that come with split rings attached to the top of the bait (where you connect to your main line). This is to improve action of the bait and allow for minimal interference. DreamWeaver is one company that does this and they have done extensive testing (I'm friends with many of them). Some spoon companies are too cheap to put on the rings or it takes too long. When running salmon boats, we NEVER put a spoon in the water without a split ring on the front. Always added our own.
SOOO, when it comes to cranks... those rings are put on for a reason. Certain baits perform differently with a ring vice without. Unlike a spoon where the ring allows for free movement (no matter the spoon), all cranks are NOT created equal. Cranks come in several shapes sizes and colors and many of them have different connecting platforms. For me, I prefer a Duo Lock for two reasons: One, it aids in being able to remove the bait quickley, Two, it allows for free movement of the crank bait. Allowing it to wobble from side to side, and tip and yawl. IF it comes with a split ring it is there for a reason. This is why I prefer a Duo Lock over a standard snap swivel. Many of the snap swivels have a smaller opening and do not allow for the the bait to wobble as designed.
For years, many people tied a standard fishermans knot directly to a Rapala HOWEVER: the box showed what was called a Rapala Knot (commonly known as a loop knot). This was provided from the manufacturor to the purchaser to SHOW them that the bait performs BEST when not impeaded by tieing direct or using tight snap swivels.
In summary, for things that rotate (IE spoons) I alwasy use a quality ball bearing swivel and split ring to allow for proper movement of the spoon and rotation without line twist. For cranks, they are not designed to rotate, I use a large mouth Duo Lock to allow the bait to move from side to side. No need for a swivel. If they are rotating, it is duo to improper tuning of the bait (most are not tuned out of the box) or issues resulting from improper line application to the reel (allowing for line twist when adding line).
I have OCD "Obsessive Crappie Disorder"