Yes, it's generally a good thing - from the FreeBSD fxp(4) man page:
Some chip revisions have loadable microcode which can be used to
reduce the interrupt load on the host cpu. Not all boards have
microcode support. Setting the link0 flag with ifconfig(8) will
download the microcode to the chip if it is available.
m0n0wall automatically sets the link0 flag on fxp NICs.