The Value of Focusing Rails (and the Welfare of Unsuspecting Ants)
It's interesting to consider what purpose a macro focusing rail could possibly serve since the lens itself already possesses the ability to change focus.
To get to the bottom of this question it helps to start with understanding how a lens focuses. And to do so, if you permit me, I want to take even one more step back to consider the popular rite of passage for young boys involving ants and a magnifying glass. It's OK to admit you did this yourself at least once growing up. You're among friends. Don't worry. We're not really going to burn any more ants today. Summer camp was a long time ago. None of us have done anything like that in years. It's just that this will serve a useful purpose for the question at hand.
But suppose for a moment we do entertain the notion that you did want to sizzle some ants peacefully going about their business down on the ground. Obviously, you need some ants on a clear sunny day, and you need a magnifying glass. If you are looking for some mischievous if perhaps needlessly cruel fun, you have to master your technique. Distance is the key. You have to find the distance that precisely focuses the sun's rays at the point of, well ... I probably don't need to come out and say it. Hold the magnifying glass too close or too far away and that ant may get a tad bit warm, but he'll live to tell his ant friends about it another day. Your magnifying glass is just like any other lens, and it has a focal length. If you really did want to achieve maximum devastation, you'd have to get the distance right.
OK, enough with maiming ants. Suppose instead your objective is to take a picture of those ants instead. You may well have tried your hand at doing this before too. And in this endeavor, your camera lens shares something in common with that fire-starting magnifying glass. Except in this case, it has to be held the correct distance from your camera's sensor to achieve focus and form a sharp image of those ants. You're not trying to focus light on the ants; you're trying to focus the light reflecting off of them onto your camera's sensor. You move the center of your lens closer or further away from the sensor by telling the camera to engage the motor that turns the lens's focusing helicoids. With many modern lenses, it may not be evident that the lens moves but it does. A lot of lenses have what is called "internal focusing" which means that the lens elements move inside the lens barrel without the front of the lens itself moving or rotating.
But in order to achieve focus by moving the lens elements closer or further away, you are also changing the magnification ratio. If you'll allow me to mention the fate of those ants again, you may remember something else regarding that magnifying glass from your summer camp experiences years ago. It makes the images of those ants bigger or smaller depending on how far away from them you hold it. It is, after all, a magnifying glass. Changing distance changes magnification. And as above, the same holds true for your camera lens. You generally won't notice the difference in magnification that focusing your camera lens causes, but it does affect it. As the center of your lens moves close or further away from your camera's sensor plane, the magnification changes along with the focus.
Geared focusing rail (top) and simpler Arca-Swiss slider rail (bottom)
You might be able to safely ignore all this if you use just a macro lens for your close-up ant photography, but if you want to attach a supplemental diopter on the front of your lens or put an extension tube between your camera and lens and things get more tricky. These sorts of lens modifiers also affect both magnification and focus.
A focusing rail avoids the entire issue by allowing you to move your camera and lens together, as a unit, to achieve focus. This lets you retains the distance relationship between lens elements and sensor plane and thereby avoids changing magnification when changing focus. The focus point of your lens/camera system remains the same distance away, but you change what is in focus by changing where that point lies. Whatever the ideal point of focus for a given subject is, a focusing rail lets you move your camera and lens together until the whole thing is the correct distance from that subject focus point.
So assuming you agree with the value of a focusing rail for macro and close-up work, it's worth considering what kind to get.
Focusing rails come in a number of different designs, but I'm an advocate of not overcomplicating things. As an equally strong advocate of the Arca-Swiss compatible clamping system, the simplest macro slider consists of nothing more than a lengthened dovetail plate. Slide the plate through the tripod head clamp and then lock it down when you get to where you need to be. Since the clamping mechanism is the same one that you trust for holding your camera in place normally, you know it will hold well. Pressure on the entire surface area of the clamp mechanism means it will be held safely and firmly.
Geared models purport to provide an easy way of moving your rig closer or further away from your subject. Turn the knob and the gears do the real work. But for the gears to turn there has to be some amount of play, and this can lead to trouble. Even when tightened down there will inherently be more potential for vibration than simpler solutions. When you try to lock the gears, only a small amount of surface is available. This makes it hard to hold the rig securely enough to prevent slippage. What you lack in surface area you would need to make up for by over-tightening the locking knob. The first focusing rail I bought years ago was big and bulky due to the included gear mechanism and never could be held solidly despite turning the locking knob as tightly as I could. Even a newer Really Right Stuff geared focusing rail I bought years later suffered from a tendency to "creep" along the geared rails unless the whole setup was kept close to the horizontal. Point the camera down to shoot from above and it was nearly impossible to hold a heavy camera in place regardless of how much that lock knob were tightened.
When I use a focusing rail, I want to be as sure as I can that my camera gear will be held solidly in place to get the sharpest images I can. I also want it held tightly to minimize any chance of a slippage accident damaging my camera or any unsuspecting ants on the ground below my camera. Hey, I care.