Infantry is most useful when paired with line artillery, which you cannot do with cavalry without getting rid of the speed bonus (which is the only reason you'd use them).
Most things are more useful with artillery support.
But your post brings up something I considered doing as the Soviet Union at one point.
With the NKVD focus and the training speed chief of staff (whose name eludes me for the moment), I considered building NKVD divisions (low width, pure INF) as CAV instead of INF. My thoughts were that CAV divisions move faster, so they could arrive in provinces much faster and reinforce to width in crucial battles in MP. If INF could be a stopgap with the NKVD focus, then CAV would be better.
But I discarded the plan when I realized that pure CAV trains much slower than pure INF, and they would cost significantly more infantry equipment
and would burden the supply regions disproportionately to their usefulness. If armored divisions are weighing down the supply network, that's one thing. If throw-way CAV divisions are forcing me to go out of supply (and thus penalizing the real divisions), then I'm shooting myself in the foot.
