Why on earth would you want to replace the original foot? It's perfectly fine.
Is there something I'm missing here?
To attach the lens to an Arca-Swiss type clamp on a tripod/monopod head. You can attach a lens plate to the bottom of the Canon foot, or just use a replacement foot with an integrated dovetail - the latter is lighter and less bulky.
Also, although not sure this is the case for the 300 II, IMO with the 600 II (and I suspect the 500 II), there is
something wrong with the Canon foot. They used the same forward-sweeping design as on the MkI versions, but the MkII is lighter, and a fair bit of that weight loss comes from the elimination of the protective meniscus lens in front of the front optical element - that makes the MkII less front heavy. When mounted on a gimbal with a heavy (1-series or gripped) body and a 2x TC, the center of mass is actually slightly behind the foot mounting point - and the Canon foot is base is well forward of that point, so the combo cannot be properly balanced on a gimbal. I've read that some people 'solve' that problem by using Canon monopod foot (included with the 600 II), but mounting it backward from the normal direction. The RRS replacement foot extends behind the mounting screws, allowing the lens+TC to be properly balanced.