Val'anyr's item level was only increased because people were stupid and based whether people could come to their ICC runs on what their GS was.
Being that Val'anyr was from Ulduar, 2 tiers below ICC, it's item level was rather low. However, what it actually did was better than many items from TOC and even ICC.
I know a holy paladin on my server that was doing HM 10 LK while still using Val'anyr. The proc effect on Val'anyr was amazing on that fight.
Just because the caster legendary will be what item level 391 (400 something, legendaries are usually a little higher in item level than the tier they are in), doesn't mean people won;t still be using it in later contents of raiding.
People were doing kara, and early portions of SSC and TK with Thunderfury and Atiesh.