If you've done a lot of raiding over the years, you know that it seems like there's always someone in your raid group who just can't get a weapon to drop. That was me, for the entirety of Wrath Naxxramas, stuck using my engineering gun until an Ulduar-25 PUG landed me the gun off Kologarn. It sucked - I literally couldn't crit high enough to pop the constructs on Ignis until I got a weapon upgrade.
So Legion is going to bring us artifact weapons, and no one will need to worry about weapon drops (because there won't be any). The various skins you can unlock and the ability to transmog the artifacts mean you aren't visually stuck with it for the whole expansion.