There's an argument that it artificially increases spreads in lit markets, since you have segregated the least toxic flow to only go to a few select players (largely Citadel and Virtu). Further, these increased spreads mean that the 'price improvement' is only price-improved against a spread that's being quoted against only the most toxic flow. However, many spreads are still close to a single tick, although this is less true as individual ticker prices have generally gotten higher.

The tick size changes are interesting as well in the context of auctions. If a claim is "you can't have an auction with sub-penny pricing so there's no improvement", well then make the tick size smaller.