I don't think the suggestion is a desire to animate on raw mesh, the challenge here is that Rapid Rig has created a requirement that all objects have to be rigged with rapid rig in order for their movement to align in the graph.
The challenge for games in particular is the creation of cognitive dissonance for facing direction. The character would be moving forward from it's neutral zero'd out position, but instead of increasing Z values, you get decreasing X values. The root control not being in World Space is irregular. The majority of rigged objects will have the Root control align with the world.
Even if users made their own simple curve based control rig, it would actually take some effort to break the basic node behavior to match the Rapid Rig alignment.
I definitely see the value of the advanced mirroring controls, but what I'd expect is for that behavior to be mediated by a group below the root that retains the non-standard orientation, but allows the controls to behave in the advanced way with object space.
The other challenge is that root motion alignment is a persistent issue that is continuously needed by our animation team. We generally avoid overly symmetrical poses for composition, and while there's moments where it saves time in blocking we move away from that state quickly. On the other hand, refining and aligning root movement is an ongoing process.
I'm curious, are you suggesting the intention is to require RapidRig roots for all object positions in the scene if you need space alignment?