alt Hacker NewsMorass, mess, broadcast/multicast, etc aside (seems more like complaints of complexity than layering), IPv4+ARP is the textbook example of a layering violation. When you do want to violate, having the L2 info in the L3 packet is still cleaner than L3 info in L2. One is a protocol carrying its own glue in itself, the other is a protocol using different protocols (per L2) to discover the glue the same way it could have itself anyways. It's certainly convenient of course, but that doesn't make it cleaner layering. It also gives a consistent answer for different L2s e.g. cellular links because of this.
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Whut?
IPv4 is an example of _correct_ layering. The hardware address is a detail that does not leak into upper layers. It's confined purely to the network layer.
In contrast, with IPv6 the whole 64/64 separation is a result of leaking the MAC address into upper protocols. Indeed, MAC was supposed to be a part of the publicly visible IPv6 addresses for hosts!
Sticking L2 into L3 means that L3 needs the ability to communicate with nodes with as-yet-unknown L2 addresses and that L3 nodes that don’t have an L3 address yet need to be able to transmit L3 packets. Both of these are quite messy, and APR completely avoids these problems.
(I am not, however, defending DHCPv4 - that has some of the same problem.)