So ramped/pinned/variably-shaped-teeth chainrings have improved front shifting significantly, and made front indexing viable if still a frequent PITA for many folks.
In nearly all instances I've seen, the smallest chainring (granny ring of a triple, inner ring of a double) has all of the teeth shaped the same. Ramps and pins are ususally to aid in shifting up to a larger chainring, or down from the larger chainring, and in both cases the special shaping of teeth, or ramps and pins, are useful on the larger chainring. Hence middle and large chainrings on triples, and large chainrings on doubles, are ramped and pinned and sometimes have variably shaped teeth, but the smallest chainring doesn't have anything of the sort.
But non-smallest chainrings can be designed specifically to shift best with the chain coming off a certain tooth on the inner ring. Which means that the tooth should be in a specific spot on the inner ring to best catch the ramp or pin to the outer ring.
Which means that while inner rings needn't have shaped teeth, ramped/pinned large rings are designed for a specific number of teeth on the inner ring, combined with specific rotational matching of that inner ring to the outer ring (so the teeth on the inner ring are in the spot which was expected when the ramps/pins on the outer ring was designed).
Now, the big question: how much does it actually matter that you use the inner ring that an outer ring was designed for? For example, a Shimano XTR 46t chainring may have been designed for its ramps and pins to shift from a 36t Shimano chainring. But will shifting be much compromised if the 46t ring is used with a 34t inner ring?
Likewise, an FSA 50t outer ring may have been designed for use with a 34t inner ring on a compact double setup, but how much worse will it shift when coming off a 36t Shimano chainring?
This is an interesting technical case where it makes sense that design differences actually matter, but it's not clear that they matter very much. Manufacturers have an incentive for chainrings to be paired, so that consumers will buy two chainrings at once instead of replacing just one. How much practical reason is there for buying chainrings that were designed as part of a system?