As I pointed out, and Steiner verified, there are existing hardware
implementation that do *not* verify the checksum.  So arguing that RFC
1812 (which isn't even a standard) requires it is a bit silly.

As for checking the length, another option might be to keep track of
the minimum packet length for a given interface, and only truncate if
the outgoing interface would not have to pad.  Right now, we *do* have
to truncate and repad Ethernet packets -- in exactly the case Jonathan
cites, even -- to send them out some interfaces.  This is clearly bad
for router performance.  (Note though that some cards do the padding
automatically.)