You haven't thought through what you're asking. That's the bad argument. Network packets are not viable without a destination address. Nor does anyone want unaddressed (garbage) packets on their network.
Network packets don't need a destination address. Broadcast addresses exist. Also, packets to invalid/unknown destinations exist. You can send network packets with invalid source or destination addresses already anyway.
Taking a raw chunk of data and putting it on the wire as-is is the most logical interpretation of "writing to the ethernet device". Does it make sense to allow everyone to do that? Certainly not, that's why you restrict access to devices anyway.
The fact that not every chunk of data "makes sense" for every device in /dev is certainly nothing new, since that is the case for all other devices already (I mentioned a few in my post above).
Packets don't need to be routed. Sometimes you just want to communicate with a host on the same Layer-2 network. I said "Broadcast" (not Multicast) on purpose.
Sometimes you don't even want TCP/IP on the wire. Heck, sometimes you maybe don't even want DIX Ethernet on the wire.
Anyway, this discussion is going nowhere. Handcrafting packets is possible (it's basically what the kernel does anyway), sometimes it's useful, and if you could write a user-space program that could just open /dev/eth0 and write its own handcrafted packets to that stream would be helpful.
