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The rationale for light a flea is the same as lighting an elephant. Two components are needed: 1) off axis "key" light to create the 3D modeling with highlight and shadow, and; 2) a fill source to fit the range of the scene to the shorter range of the sensor.
The convention of putting a key light in front at 45 degrees evolved to meet the unique requirements of modeling human faces in a flattering way. That requires getting light into the recessed eye sockets and controlling how the sundial nose in between casts it's shadow. Most macro subjects don't have recessed eyes with a sundial nose between them and don't need to be illuminated that way. To the contrary, the strongest illusion of 3D in a 2D photo is created by shooting into the shadow side of an object, with the key light behind and to the side at about 135 degrees from the camera axis with fill coming from the direction of the camera.
Flat fill is ideal because sideway fill cast shadows and where there are fill shadow and also no key light there will be no detail. Because lens - subject distances can be quite small a problem with Macro is how to get fill between the lens and the subject without the lens casting a shadow. That's not as much of a problem with a long macro like the 100mm.
SLR ring flash was originally designed for shadowless illumination when documenting medical / dental procedures inside body cavities and mouths, but a single fiat light isn't the ideal for creative photography. However the MR-14 is an ideal fill source in a two light scenario because it can serve as a Master for a conventional Canon EX flash used as key light behind and to the side of the subject.
The MR-24 is a self-contained two light solution which is ideal because many macro situations don't lend themselves to using an off camera flash on a stand as key light. But the default configuration will cross light subjects. Cross lighting can be used effectively, but can result in unfilled voids where neither light reaches.
If you already have Canon flashes here's a DIY alternative to consider. I already owned a pair of 580ex flashes and when I decided to try macro with a set of extension tubes I devised a DIY macro solution which would give me fill from the just over the top of the lens with my 580ex master flash mounted in the hotshoe, used with a second off camera 580ex as key light. See it here: LINK
Used by itself the diffuser on the camera creates lighting with natural looking downward modeling. The modeling looks natural because natural light comes from overhead most of the time and our brain is programmed to recognize shape based on contrast patterns of shadow and highlight created from overhead sources. Adding the off camera flash from behind and the side adds modeling which enhanced the illusion of 3D.