racking . . . sticking . . . unpleasant other things . . .
or, one can use just a single miter slot guide.
well, tacks come to brass,,,, (typically) the right side of the sled does nothing except support the cut-off piece in the vertical plane 'until the bitter end' - whether the right side is perfectly square to a 0.001 degree is completely irrelevant - the right side cut-off is just floating along until the last fiber is severed. yes, there is also front-to-back / lateral stability provided by the rail extended to the right of the cut.... which is likely important if you're trying to cut a 5' length _exactly_ right on the first-and-only-pass. which I never indulge myself in.... I do a 'rough' cut then nibble it down to the precise dimension.
then again, I've been labelled weird . . .
a sled with a back and leading rail provides stability to the two flat sled pieces.
whether it's a single miter or both sides miter slot design.
in the photos above, do note the dimensions of the back rail for the 'single rail' sled options - especially the thick/wide back rail. glued down that makes for some serious stiffness from the left-sled-flat to the right-sled-flat.
the back + front rail design does has an advantage when the sled is dismounted from the saw table. seriously less 'flapping-in-the-wind' with a back + front design.