Swimming is first and foremost about getting "traction" i.e. maximizing the amount of force you can apply without your hands moving backwards through the water. At a basic level, a high turnover/cadence signifies that you are slipping your hands through the water and not getting forward motion out of that wasted energy.
To be a bit more precise, you want to maximize the force applied opposite the direction you wish to go. Putting a lot of force into a sideways motion (the old "s-stroke") is wasting energy.
Also, the force generated is integrated over the time spent. So if you slip your hands through the water at a point in the stroke in which the propulsive force is minimal (think at the end of the pull), thereby not getting forward motion, but this allows you to return to the maximal propulsive force portion of the stroke more quickly, you *can* increase the effective force applied over time. Same applies if your stroke is a little less effective but you get more of them.
If the effectiveness of the rest of your pull is high, then slipping when using a high turnover can make you faster. In addition, the maximal torque applied during the stroke may be lessened, and some people (like me) can go faster and longer with lower torque and higher cadence (both in the water and on the bike). There's a reason why nearly all the bike hour records were set at around the same 100+ cadence (http://www.wolfgang-menn.de/hourrec.htm
Each swimmer needs to find their own sweet spot.
The old S stroke is not applying forces sideways unless you angle your hands in a direction to create vector forces in that direction.
Swimmers who lack a grip on the water often simply do not have enough sideways motion and too much backward motion which is very heavily drag dominated and very inefficient.
Paddle wheels are not as efficient as propellers.
The S stroke is the propeller motion.
Moving your limbs sideways to the direction of motion is exactly what a propeller does, they don't move backwards as once the water surrounding them starts to move back there is nothing left to push against. You need fresh water to push against at all times.
Try treading water with your arms by pushing downwards with your hands and not moving sideways in a sculling motion and you will quickly discover how people who cannot swim drown.
That sideways motion is exactly what the S stroke is, 3 sculling actions joined as efficiently as possible together to create a single stroke.
Even a huge blade used by a kayaker is not pulled backwards, is is put in close to the boat and moves outward until it is exited. At no time during the stroke does it slip backward in the water, it sculls sideways and to an extent downwards at the beginning of the stroke, just like a swimmer.
During sprinting the drag component may increase, just like an aeroplane coming into land.
When landing, an aeroplane's wings which have been optimised for efficient 'in flight' use do not offer enough lift at slow landing speeds so the plane is flown nose up to create extra lift at the sacrifice of a huge increase of drag.
You can do this with your hands by angling them more and sprinters do this if their grip on the water is not enough to support their power.
So at that point you have a choice, move your arms faster to create more grip or angle your hands inefficiently to create more grip.
Moving the limbs faster by increasing turnover is more efficient than angling hands that create more drag.
This is nothing to do with a poor swimmer moving their arms faster because they lack any reasonable purchase on the water and everything to do with the best swimmers trying to find an efficient way to get huge amounts of power down.
If you consider limb/hand speed, which is akind to cycling cadence, then you will see that what you think are swimmers with slow turnovers are actually swimmers with very high limb speed due to a lot of sideways movement (or long arms), just the same as a cyclist using a longer crank and having higher foot speed at the same cadence as a shorter crank with a higher cadence.
It is the limb speed that creates the support for the power supplied and everybody has their own way of achieving that.
Those with larger hands can get enough support at lower hand speeds, those with smaller hands and similar power output need higher hand speed.
So depending on your makeup, your stroke will have to take into consideration how much limb speed is required for the amount of power you have to get transferred into body motion and some folks stroke rate will vary wildly to others because of their power output, limb dimensions, weight to be accelerated and body drag which are all individual.