The platform cc2538dk might not set the 64-bit address when LINKADDR_SIZE != 8 right now but a different platform with cc2538 might. Or at least could if the set_object() in the radio driver would allow it ;-)

Setting the 64-bit address makes a difference if the radio receives a packet with a 64-bit address even when LINKADDR_SIZE != 8 as IEEE 802.15.4 supports both 16 bit and 64 bit addresses simultaneously. So personally, I would prefer if the radio drivers allow getting/setting the 64 bit address regardless of what LINKADDR_SIZE is set to.